http://en.wikipedia.org/wiki/Nuclear_medicine
Nuclear medicine
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A nuclear medicine lung exam
Shown above is the bone scintigraphy of a young woman, with a pathological lesion visible under the right orbit (eye).
Nuclear medicine is a branch of medicine and medical imaging that uses the nuclear properties of matter in diagnosis and therapy. More specifically, nuclear medicine is a part of molecular imaging because it produces images that reflect biological processes that take place at the cellular and subcellular level.
Contents
[hide]
1 Background
2 Diagnostic testing
3 Types of studies
4 Analysis
5 Radiation dose
6 See also
7 References
8 External links
//
[edit] Background
Para-sagittal MRI of the head in a patient with benign familial macrocephaly. MRI are frequently performed prior to neurosurgery.
Nuclear medicine procedures use pharmaceuticals that have been labeled with radionuclides (radiopharmaceuticals). In diagnosis, radioactive substances are administered to patients and the radiation emitted is detected. The diagnostic tests involve the formation of an image using a gamma camera or positron emission tomography, invented by Hal O. Anger, and sometimes called an Anger gamma camera. Imaging may also be referred to as radionuclide imaging or nuclear scintigraphy. Other diagnostic tests use probes to acquire measurements from parts of the body, or counters for the measurement of samples taken from the patient.
In therapy, radionuclides are administered to treat disease or provide palliative pain relief. For example, administration of Iodine-131 is often used for the treatment of thyrotoxicosis and thyroid cancer. Phosphorus-32 was formerly used in treatment of polycythemia vera. Those treatments rely on the killing of cells by high radiation exposure, as compared to diagnostics in which the exposure is kept as low as reasonably achievable (ALARA policy) so as to reduce the chance of creating a cancer.
Nuclear medicine differs from most other imaging modalities in that the tests primarily show the physiological function of the system being investigated as opposed to traditional anatomical imaging such as CT or MRI. In some centers, the nuclear medicine images can be superimposed, using software or hybrid cameras, on images from modalities such as CT or MRI to highlight the part of the body in which the radiopharmaceutical is concentrated. This practice is often referred to as image fusion or co-registration.
Nuclear medicine diagnostic tests are usually provided by a dedicated department within a hospital and may include facilities for the preparation of radiopharmaceuticals. The specific name of a department can vary from hospital to hospital, with the most common names being the nuclear medicine department and the radioisotope department.
About two thirds of the world's supply of medical isotopes are produced at the Chalk River Laboratories in Chalk River, Ontario, Canada. The Canadian Nuclear Safety Commission ordered the reactor to be shut down on November 18, 2007 to facilitate repairs after safety concerns. The repairs took longer than expected and in December 2007 a critical shortage of medical isotopes occurred. The Canadian government passed emergency legislation, allowing the reactor to re-start on 16 December 2007, and production of medical isotopes to continue.[1]
The Chalk River reactor is used to irradiate materials with neutrons which are produced in great quantity during the fission of the U-235, which neutrons change the nucleus of the irradiated material by adding a neutron. For example, the second most commonly used radionuclide is Tc-99m, following the most commonly used radionuclide, F-18 (which is produced by accelerator bombardment of O-18 with protons. The O-18 constitutes about 0.20% of ordinary oxygen (mostly O-16), from which it is extracted; see FDG).
In a reactor, one of the fission products of uranium is Molybdenum-99 which is extracted and shipped to radiopharmaceutical houses all over North America. The Mo-99 radioactively beta decays with a half-life of 2.7 days, turning initially into Tc-99m, which is then extracted (milked) from a "Moly cow" (see technetium-99m generator). The Tc-99m then further decays, while inside a patient, releasing a gamma photon which is detected by the gamma camera. It decays to its ground state of Tc-99, which is relatively non-radioactive compared to Tc-99m.
[edit] Diagnostic testing
Diagnostic tests in nuclear medicine exploit the way that the body handles substances differently when there is disease or pathology present. The radionuclide introduced into the body is often chemically bound to a complex that acts characteristically within the body; this is commonly known as a tracer. In the presence of disease, a tracer will often be distributed around the body and/or processed differently. For example, the ligand methylene-diphosphonate (MDP) can be preferentially taken up by bone. By chemically attaching technetium-99m to MDP, radioactivity can be transported and attached to bone via the hydroxyapatite for imaging. Any increased physiological function, such as due to a fracture in the bone, will usually mean increased concentration of the tracer. This often results in the appearance of a 'hot-spot' which is a focal increase in radio-accumulation, or a general increase in radio-accumulation throughout the physiological system. Some disease processes result in the exclusion of a tracer, resulting in the appearance of a 'cold-spot'. Many tracer complexes have been developed in order to image or treat many different organs, glands, and physiological processes. The types of tests can be split into two broad groups: in-vivo and in-vitro:
[edit] Types of studies
A typical nuclear medicine study involves administration of a radionuclide into the body by intravenous injection in liquid or aggregate form, ingestion while combined with food, inhalation as a gas or aerosol, or rarely, injection of a radionuclide that has undergone micro-encapsulation. Some studies require the labeling of a patient's own blood cells with a radionuclide (leukocyte scintigraphy and red blood cell scintigraphy). Most diagnostic radionuclides emit gamma rays, while the cell-damaging properties of beta particles are used in therapeutic applications. Refined radionuclides for use in nuclear medicine are derived from fission or fusion processes in nuclear reactors, which produce radioisotopes with longer half-lives, or cyclotrons, which produce radioisotopes with shorter half-lives, or take advantage of natural decay processes in dedicated generators, i.e. molybdenum/technetium or strontium/rubidium.
The most commonly used intravenous radionuclides are:
Technetium-99m (technetium-99m)
Iodine-123 and 131
Thallium-201
Gallium-67
Fluorine-18 Fluorodeoxyglucose
Indium-111 Labeled Leukocytes
The most commonly used gaseous/aerosol radionuclides are:
Xenon-133
Krypton-81m
Technetium-99m Technegas
Technetium-99m DTPA
[edit] Analysis
The end result of the nuclear medicine imaging process is a "dataset" comprising one or more images. In multi-image datasets the array of images may represent a time sequence (ie. cine or movie) often called a "dynamic" dataset, a cardiac gated time sequence, or a spatial sequence where the gamma-camera is moved relative to the patient. SPECT (single photon emission computed tomography) is the process by which images acquired from a rotating gamma-camera are reconstructed to produce an image of a "slice" through the patient at a particular position. A collection of parallel slices form a slice-stack, a three-dimensional representation of the distribution of radionuclide in the patient.
The nuclear medicine computer may require millions of lines of source code to provide quantitative analysis packages for each of the specific imaging techniques available in nuclear medicine.
Time sequences can be further analysed using kinetic models such as multi-compartment models or a Patlak plot.
[edit] Radiation dose
A patient undergoing a nuclear medicine procedure will receive a radiation dose. Under present international guidelines it is assumed that any radiation dose, however small, presents a risk. The radiation doses delivered to a patient in a nuclear medicine investigation present a very small risk of inducing cancer. In this respect it is similar to the risk from X-ray investigations except that the dose is delivered internally rather than from an external source such as an X-ray machine.
The radiation dose from a nuclear medicine investigation is expressed as an effective dose with units of sieverts (usually given in millisieverts, mSv). The effective dose resulting from an investigation is influenced by the amount of radioactivity administered in megabecquerels (MBq), the physical properties of the radiopharmaceutical used, its distribution in the body and its rate of clearance from the body.
Effective doses can range from 6 μSv (0.006 mSv) for a 3 MBq chromium-51 EDTA measurement of glomerular filtration rate to 37 mSv for a 150 MBq thallium-201 non-specific tumour imaging procedure. The common bone scan with 600 MBq of technetium-99m-MDP has an effective dose of 3 mSv (1).
Formerly, units of measurement were the Curie (Ci), being 3.7E10 Bq, and also 1.0 grams of Radium (Ra-226); the Rad (radiation absorbed dose), now replaced by the Gray; and the Rem (Rad Equivalent Man), now replaced with the Sievert. The Rad and Rem are essentially equivalent for almost all nuclear medicine procedures, and only alpha radiation will produce a higher Rem or Sv value, due to its much higher Relative Biological Effectiveness (RBE). Alpha emitters are nowadays rarely used in nuclear medicine, but were used extensively before the advent of nuclear reactor and accelerator produced radioisotopes. The concepts involved in radiation exposure to humans is covered by the field of Health Physics.
[edit] See also
Chalk River Laboratories
Dose calibrator
Gamma camera
MAG3 scan
Medical imaging
Positron emission tomography
Radiology
Radionuclide
Radionuclide cisternogram
Radiophobia
SPECT
Technetium-99m generator
Therac-25
Radiological protection of patients
Wednesday, October 29, 2008
SPECT CT
http://en.wikipedia.org/wiki/SPECT
Single photon emission computed tomography
From Wikipedia, the free encyclopedia
(Redirected from SPECT)
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Single photon emission computed tomography (SPECT, or less commonly, SPET) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it is able to provide true 3D information. This information is typically presented as cross-sectional slices through the patient, but can be freely reformatted or manipulated as required.
A Lung SPECT / CT Fusion image
Contents
[hide]
1 Principles
2 Application
2.1 Myocardial perfusion imaging
2.2 Functional brain imaging
3 Reconstruction
4 Further reading
5 Typical SPECT acquisition protocols
6 See also
7 External links
//
[edit] Principles
In the same way that a plain X-ray is a 2-dimensional (2-D) view of a 3-dimensional structure, the image obtained by a gamma camera is a 2-D view of 3-D distribution of a radionuclide.
SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (also called projections), from multiple angles. A computer is then used to apply a tomographic reconstruction algorithm to the multiple projections, yielding a 3-D dataset. This dataset may then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other tomographic techniques, such as MRI, CT, and PET.
Because SPECT acquisition is very similar to planar gamma camera imaging, the same radiopharmaceuticals may be used. If a patient is examined in another type of nuclear medicine scan but the images are non-diagnostic, it may be possible to proceed straight to SPECT by moving the patient to a SPECT instrument, or even by simply reconfiguring the camera for SPECT image acquisition while the patient remains on the table.
To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3-6 degrees. In most cases, a full 360 degree rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15 – 20 seconds is typical. This gives a total scan time of 15-20 minutes.
Multi-headed gamma cameras can provide accelerated acquisition. For example, a dual headed camera can be used with heads spaced 180 degrees apart, allowing 2 projections to be acquired simultaneously, with each head requiring 180 degrees of rotation. Triple-head cameras with 120 degree spacing are also used.
Cardiac gated acquisitions are possible with SPECT, just as with planar imaging techniques such as MUGA. Triggered by Electrocardiogram (EKG) to obtain differential information about the heart in various parts of its cycle, gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion, thickness, and contractility of the myocardium during various parts of the cardiac cycle; and also to allow calculation of left ventricular ejection fraction, stroke volume, and cardiac output.
[edit] Application
SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful. E.g. tumor imaging, infection (leukocyte) imaging, thyroid imaging or bone imaging.
Because SPECT permits accurate localisation in 3D space, it can be used to provide information about localised function in internal organs. E.g. functional cardiac or brain imaging.
[edit] Myocardial perfusion imaging
Myocardial perfusion imaging (MPI) is a form of functional cardiac imaging, used for the diagnosis of ischemic heart disease. The underlying principle is that under conditions of stress, diseased myocardium receives less blood flow than normal myocardium. MPI is one of several types of cardiac stress test.
A cardiac specific radiopharmaceutical is administered. E.g. 99mTc-tetrofosmin (Myoview, GE healthcare), 99mTc-sestamibi (Cardiolite, Bristol-Myers Squibb). Following this, the heart rate is raised to induce myocardial stress, either by exercise or pharmacologically with adenosine, dobutamine or dipyridamole (aminophylline can be used to reverse the effects of dipyridamole).
SPECT imaging performed after stress reveals the distribution of the radiopharmaceutical, and therefore the relative blood flow to the different regions of the myocardium. Diagnosis is made by comparing stress images to a further set of images obtained at rest. As the radionuclide redistributes slowly, it is not usually possible to perform both sets of images on the same day, hence a second attendance is required 1-7 days later (although, with a Tl-201 myocardial perfusion study with dipyridamole, rest images can be acquired as little as two-hours post stress). However, if stress imaging is normal, it is unnecessary to perform rest imaging, as it too will be normal – thus stress imaging is normally performed first.
MPI has been demonstrated to have an overall accuracy of about 83% (sensitivity: 85%; specificity: 72%) [1], and is comparable (or better) than other non-invasive tests for ischemic heart disease, including stress echocardiography.
[edit] Functional brain imaging
Usually the gamma-emitting tracer used in functional brain imaging is 99mTc-HMPAO (hexamethylpropylene amine oxime). 99mTc is a metastable nuclear isomer which emits gamma rays which can be detected by a gamma camera. When it is attached to HMPAO, this allows 99mTc to be taken up by brain tissue in a manner proportial to brain blood flow, in turn allowing brain blood flow to be assessed with the nuclear gamma camera.
Because blood flow in the brain is tightly coupled to local brain metabolism and energy use, the 99mTc-HMPAO tracer (as well as the similar 99mTc-EC tracer) is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia. Meta analysis of many reported studies suggests that SPECT with this tracer is about 74% sensitive at diagnosing Alzheimer's disease, vs. 81% sensitivity for clinical exam (mental testing, etc.). More recent studies have show accuracy of SPECT in Alzheimer diagnosis as high as 88% PMID 16785801. In meta analysis, SPECT was superior to clinical exam and clinical criteria (91% vs. 70%) in being able to differentiate Alzheimer's disease from vascular dementias. PMID 15545324 This latter ability relates to SPECT's imaging of local metabolism of the brain, in which the patchy loss of cortical metabolism seen in multiple strokes differs clearly from the more even or "smooth" loss of non-occipital cortical brain function typical of Alzheimer's disease.
99mTc-HMPAO SPECT scanning competes with FDG PET scanning of the brain, which works to assess regional brain glucose metabolism, to provide very similar information about local brain damage from many processes. SPECT is more widely available, however, for the basic reason that the radioisotope generation technology is longer-lasting and far less expensive in SPECT, and the gamma scanning equipment is less expensive as well. The reason for this is that 99mTc (technetium-99m) is extracted from relatively simple technetium-99m generators which are delivered to hospitals and scanning centers weekly, to supply fresh radioisotope, whereas FDG PET relies on FDG which must be made in an expensive medical cyclotron and "hot-lab" (automated chemistry lab for radiopharmaceutical manufacture), then must be delivered directly to scanning sites, with delivery-fraction for each trip handicapped by its natural short 110 minute half-life.
[edit] Reconstruction
Reconstructed images typically have resolutions of 64x64 or 128x128 pixels, with the pixel sizes ranging from 3-6 mm. The number of projections acquired is chosen to be approximately equal to the width of the resulting images. In general, the resulting reconstructed images will be of lower resolution, have increased noise than planar images, and be susceptible to artifacts.
Scanning is time consuming, and it is essential that there is no patient movement during the scan time. Movement can cause significant degradation of the reconstructed images, although movement compensation reconstruction techniques can help with this. A highly uneven distribution of radiopharmaceutical also has the potential to cause artifacts. A very intense area of activity (e.g. the bladder) can cause extensive streaking of the images and obscure neighboring areas of activity. (This is a limitation of the filtered back projection reconstruction algorithm. Iterative reconstruction is an alternative algorithm which is growing in importance, as it is less sensitive to artifacts and can also correct for attenuation and depth dependent blurring).
Attenuation of the gamma rays within the patient can lead to significant underestimation of activity in deep tissues, compared to superficial tissues. Approximate correction is possible, based on relative position of the activity. However, optimal correction is obtained with measured attenuation values. Modern SPECT equipment is available with an integrated x-ray CT scanner. As X-ray CT images are an attenuation map of the tissues, this data can be incorporated into the SPECT reconstruction to correct for attenuation. It also provides a precisely registered CT image which can provide additional anatomical information.
[edit] Further reading
Elhendy et al., Dobutamine Stress Myocardial Perfusion Imaging in Coronary Artery Disease, J Nucl Med 2002 43: 1634-1646
For anyone interested in the brain-imaging applications of SPECT then this is a great review, although the full-text article is not available online without a subscription to the journal. The following link provides a link to the abstract only, and you might be able to access the full article through a membership with a medical library. W. Gordon Frankle, Mark Slifstein, Peter S. Talbot, and Marc Laruelle (2005). "Neuroreceptor Imaging in Psychiatry: Theory and Applications". International Review of Neurobiology, 67: 385-440.
[edit] Typical SPECT acquisition protocols
Study
Radioisotope
Emission energy (keV)
Half-life
Radiopharmaceutical
Activity (MBq)
Rotation (degrees)
Projections
Image resolution
Time per projection (s)
Bone scan
Technetium-99m
140
6 hours
Phosphonates / Bisphosphonates
800
360
120
128 x 128
-
Myocardial perfusion scan
Technetium-99m
140
6 hours
tetrofosmin; Sestamibi
700
180
60
128 x 128
30
Brain scan
Technetium-99m
140
6 hours
HMPAO; ECD
555-1110
360
64
128 x 128
30
Tumor scan
Iodine-123
159
13 hours
MIBG
400
360
60
64 x 64
30
White cell scan
Indium-111 & Technetium-99M
171 & 245
67 hours
in vitro labelled leucocytes
18
360
60
64 x 64
30
[edit] See also
Gamma camera
Neuroimaging
Functional neuroimaging
Magnetic resonance imaging
Positron emission tomography
ISAS (Ictal-Interictal SPECT Analysis by SPM)
http://www.dimag.com/techfocus/SPECT_CT/2006jun/
SPECT/CT settles into variety of clinical practice settings
Contributions to cardiology, oncology, and infection imaging continue, while potential applications emerge from unexpected directions
By: Paula Gould
SPECT/CT has been tagged, rather unkindly, as a modality solution looking for a problem. But as early adopters are showing, it has considerable potential as an all-rounder in busy nuclear medicine departments.
Nuclear cardiology is a prime example. All SPECT images are subject to a certain degree of spatial distortion, caused by differing scatter and absorption of emitted photons before they reach the detector. Correction for this likely attenuation is especially important in cardiac SPECT, where soft-tissue attenuation artifacts can become confused with perfusion defects. Such artifacts can also mask signs of coronary artery disease.
Attenuation correction can be performed relatively easily on hybrid scanners, using anatomic maps derived from CT. The results are impressive, according to Dr. Robert Iwanochko, an assistant professor of medicine at Toronto Western Hospital, who started using SPECT/CT in the hospital's cardiac center this spring.
"We look at all the information, both corrected and uncorrected, so we can see a clear advantage using CT for attenuation correction," he said. "Transmission-based correction is dying. That technology will probably not even be available in two or three more years."
Approximately 30 patients a day undergo SPECT at Toronto Western's cardiac center. One-third of these will be directed onto the hybrid scanner. Patients with a high body mass index are usually directed to the SPECT/CT system, given that attenuation artifacts are theoretically worse in obese subjects.
Iwanochko is keen to test how SPECT/CT fares against a top-class rival. The gold standard for attenuation correction in perfusion imaging at present is rubidium-82 PET, he said. A head-to-head comparison between the two techniques is likely once the planned installation of a new PET scanner at the cardiac center has been completed.
For now, however, Iwanochko is concentrating on building experience with SPECT/CT. Time will tell whether the hybrid scanner lives up to its promise of improving diagnostic confidence.
"The 'equivocal/probable' category tends to shrink when you have good attenuation correction, and that's what we are hoping for," he said. "Patients are either assigned a defect on the basis of SPECT imaging, or they are assigned to be normal."
The 16-slice SPECT/CT system up and running since July 2005 at Baptist Hospital in Miami was purchased primarily with cardiac applications in mind. Approximately 60% of the workload is now cardiac SPECT/CT, according to Dr. Jack Ziffer, chief of radiology. A second system is due to come online once room renovations are complete, which will ease demands for time on the hybrid scanner.
Attenuation correction for myocardial perfusion imaging has proven to be robust, Ziffer said. Prone scans are now unnecessary in nearly all cases, leading to faster examinations. Diagnostic-quality CT also permits coronary calcium scoring to be carried out while patients are still on the table. If calcium scores indicate a low risk of myocardial infarction, patients may then be triaged home without the need for stress testing.
Achieving a good match between the CT and SPECT images was not automatic, Ziffer said. CT data for attenuation correction had previously been acquired on a nondiagnostic scanner during respiration. The images may have been blurred, but at least they fit well with the cardiac SPECT data, which were also acquired during respiration. Moving to 16-slice CT raised the question of exactly when to image: during slow breathing, inspiration, or expiration?
"We have learned that end-tidal expiration is the best time to do the CT," Ziffer said. "We still use a software application to align the heart with the mediastinum, because sometimes the nuclear and CT images are not exact. That's an important piece for getting good cardiac attenuation correction."
The six-slice SPECT/CT system acquired by The Cleveland Clinic Foundation is similarly being used for both attenuation correction and calcium scoring during cardiac examinations. Time for cardiac studies is limited, however, given competing demands on the system from other specialties, said Dr. Donald Neumann, director of molecular oncologic applications. The real advantages of hybrid imaging are crucial when it comes to scheduling.
"We have tended to focus on larger patients, with the hope of removing some of the problems associated with attenuation artifacts," he said. "But it is premature at this point to say which cardiac patient is going to benefit most from that unit."
LOCALIZING PATHOLOGY
Many neurologic applications are also likely to reap benefits from SPECT/CT through superior attenuation correction, according to Neumann. Where oncology can gain is through anatomic localization.
Nuclear medicine techniques are adept at seeking out areas of malignancy. The growing armamentarium of radioisotope tracers is also widening options for cancer diagnosis, staging, and follow-up far beyond FDG-PET. The poor spatial resolution of nuclear medicine images can make it difficult to pinpoint the exact location of pathology, however, with important implications for therapy planning and the use of imaging to monitor treatment efficacy.
Overlaying PET or SPECT images with CT data can provide the ideal combination of functional and anatomic information in a single view. If both sets of data have been collected in the same examination, making a good match is likely to be much easier.
Patients undergoing indium-111 octreotide studies, meta-iodobenzylguanidine (MIBG) studies for neuroblastoma, and In-111 Prostascint for prostate antibody imaging are all routinely scheduled onto the Cleveland Clinic's SPECT/CT unit. Cases of hyperparathyroid tumors and suspected hepatic hemangiomas are evaluated on the hybrid system as well.
"On occasion, we also get a call for evaluation of an abdominal mass, with the suspicion of a possible accessory spleen or splenic remnant, and those will definitely be performed on the SPECT/CT, too," Neumann said.
The improved capacity for anatomic localization has yielded additional benefits in lymphoscintigraphy.1 The lymphatic drainage pattern is often ambiguous in patients with head and/or neck melanomas, he said. Surgeons looking for help in localizing the sentinel lymph nodes have found that coregistered CT and SPECT images provide the information they require.
"I used to be able to say 'I think there's a sentinel lymph node in the right neck.' Now I can tell 'There are three sentinel lymph nodes: One is at right level Ib, the second one is at level Va, and the third is at level III on the right," Neumann said.
Sentinel lymph node mapping is also much quicker when using a single SPECT/CT unit, according to Dr. Shahid Mahmood, clinical director of Mount Elizabeth Hospital in Singapore. Mount Elizabeth's nuclear medicine and PET center receives patients from Malaysia, Indonesia, and Thailand as well as from Singapore itself. The new six-slice SPECT/CT unit, which came online in September 2005, is also being used in diagnostic oncology to improve the throughput and accuracy of octreotide studies and parathyroid imaging.
Dr. Homer Macapinlac, chair of nuclear medicine at M.D. Anderson Cancer Center in Houston, was so confident that SPECT/CT would benefit the center's oncology practice that he supported investment in five new multislice hybrid scanners. All systems came online together for clinical work at the start of this year. Now physicians are working to gain experience with the protocols they have instituted.
One prime area of use is likely to be screening for bone metastases in patients with breast or prostate cancer. Nuclear medicine bone scans, on their own, can often suggest that patients are getting worse, Macapinlac said. So-called flare phenomena can be misinterpreted as metastatic progression when, in fact, patients are responding to treatment. Performing CT with SPECT could confirm this by showing whether lesions are becoming more calcified. This finding would then remove the need for a follow-up bone scan in three to six months and reduce unnecessary patient anxiety.
"This could make us more efficient and effective, in diminishing the number of times we scan to try and evaluate whether these patients are getting better or worse," Macapinlac said. "Having CT together with SPECT should make this a stronger modality for evaluating metastatic disease."
Other diagnostic oncology applications at M.D. Anderson include In-111 octreotide for neuroendocrine tumors, I-123 MIBG imaging of neuroblastomas in pediatric patients, and presurgical localization of parathyroid adenomas using technetium-99m 2-methoxy isobutyl isonitrile (MIBI).
"I now ask myself why we never had these machines before," Macapinlac said. "The additive information from the SPECT and CT together make them potentially more powerful and accurate than the separate studies. SPECT contributes to the interpretation of the CT study, and vice versa."
Another issue physicians must grapple with is how to deal with patients whose scans may have been misdiagnosed in the past, Macapinlac said. Previous reports based on separate SPECT and CT scans may have failed to identify true malignancies and/or benign lesions because findings appeared ambiguous.
"With the better imaging we now have, we are trying to update the status of our patients in the best way that we can," he said. "It's not just identification of disease but also the identification of what is benign. That can be more important than the identification of the cancer itself."
Most SPECT/CT studies performed at Maasland Hospital in Sittard, the Netherlands, are likewise related to oncology applications. The hybrid system, which has an integrated dual-slice CT scanner, came online for clinical work in August 2005.
All patients suspected of having bone metastases are now scheduled for a SPECT/CT scan. This helps differentiate true metastases from, for instance, signs of trauma or degenerative joint disease, said Dr. Paul Thimister, a nuclear physician at Maasland. Meanwhile, sentinel lymph node imaging for breast cancer patients and imaging of patients with parathyroid tumors are proving to be of considerable use to the surgical team.
"We have seen some very small parathyroid tumors, and with the combined SPECT/CT, we were able to locate the node exactly. We had two patients in which this was in the mediastinal region, so it was very important for the surgeon to know whether he had to perform a sternotomy to reach the node," Thimister said.
Upgrading to SPECT/CT has made presurgical assessment of parathyroid adenomas a great deal easier, according to Ziffer. Prior to installation of the hybrid scanner, patients first underwent planar imaging and SPECT with Tc-99m MIBI. Adorned with appropriately placed radiopaque markers, they would then be moved to a separate unit for neck CT.
The strategy provided surgeons with sufficient anatomic detail that they could perform accurate parathyroid adenoma surgery extremely quickly, Ziffer said. But nuclear physicians and patients had to cope with a relatively cumbersome procedure. Now all imaging can be performed on a single machine, removing the need for markers and movement between scanners.
"This has streamlined things and made image registration much more exact," Ziffer said.
At New York City's Lenox Hill Hospital, oncology applications have taken a back seat to other uses. Here the year-old 16-slice SPECT/CT system is more often used to seek out signs of infection, using the radioisotopes In-111 and gallium-67. As with oncology, infection imaging is another area of nuclear medicine that can benefit from the greater anatomic localization provided by SPECT/CT. More accurate identification of infection sites can assist considerably with treatment planning.
ORTHOPEDIC OPTIONS
The biggest impact of SPECT/CT at Lenox Hill, however, has been in orthopedics, according to Dr. Stephen Scharf, chief of nuclear medicine. Scharf is using the hybrid system to improve presurgical evaluation of the large number of patients referred to nuclear medicine with chronic foot pain. Surgery is generally regarded as a last resort for these patients. Bone scans are typically performed prior to intervention, so surgeons can double-check that their planned strategy is targeting the correct area.
"When we moved to doing this patient group on SPECT/CT, we found that the detail we could give the surgeons was in some cases sufficient to change their mind about whether to do surgery," Scharf said. "It gives us tremendous anatomic detail and allows us to characterize pathology in a way that we have never been able to do before."
A considerable number of arthritic patients with chronic pain in the arches of the foot, or the midfoot, will benefit from drastic surgical intervention such as bone fusion, Scharf said. But intervention may not be the best option for patients whose pain is actually due to an occult fracture or past traumatic episode. SPECT/CT helps differentiate between these two patient groups by distinguishing focal activity along a joint line from activity within the bone itself.
SPECT/CT changed the management of one of the first foot pain patients scanned with it, he said. Imaging sowed sufficient doubts about the wisdom of bone fusion that the surgeon opted instead to administer a local anesthetic and steroids under fluoroscopy guidance. One year later, the patient has yet to experience any recurrence of pain.
The combination of functional and anatomic data may also aid assessment of patients scheduled for spinal surgery, Scharf said. Experience from a handful of patients scheduled for vertebroplasty has highlighted the significance of seeing anatomy in more detail. Vertebroplasty can be an extremely effective way of alleviating pain caused by disc degeneration. But surgeons are unlikely to cement all abnormal discs lest procedures become too long, too expensive, and subject to greater risks. Morphing CT and SPECT data can indicate exactly where the needle should be placed.
"We had one dramatic case in which it was clear that if we had left the surgeon to his own devices, he would have chosen the wrong one," Scharf said. "The abnormality on the bone scan was right at the junction of two vertebrae. The one that was collapsed turned out not to be the one that was abnormal."
He acknowledges that orthopedic imaging is somewhat of a bonus area for SPECT/CT. Little if anything had been said on this topic when the hybrid systems were launched. One reason is likely to be the rise of MRI for musculoskeletal imaging, especially in the U.S. In the face of such competition, many orthopedists have abandoned bone scanning for all but a few specialized areas, Scharf said.
"Around here, I've got 10 MR scanners within 10 blocks," he said. "But now the foot surgeons have started to come back and look at this as an interesting technology, and we are starting to do all of our vertebroplasty patients on SPECT/CT."
Dr. Paul Shreve, a radiologist with PET Medical Imaging Center and Advanced Radiology Services in Grand Rapids, MI, has similarly identified vertebroplasty as an important application for SPECT/CT. He hopes to start working with a hybrid scanner later this year.
"Sometimes it is hard to figure out which vertebral body is hot on the bone scan, and what it corresponds to on the CT scan. With SPECT/CT, we will be able do both examinations at once, and everything will be aligned correctly," he said.
In Europe, where MRI has yet to exert as strong a grip on musculoskeletal imaging, early adopters of SPECT/CT have been quick to spot potential benefits in this area. The first patient imaged with SPECT/CT at Maasland Hospital demonstrated the potential of hybrid scanning to orthopedic applications. The patient was suffering from low back pain, and after bone scintigraphy, clinicians suspected osteomyelitis. But CT data from the region of interest identified on SPECT revealed a fracture in the lower spine and spinal compression. Instead of receiving antibiotics, as had been planned, the patient was scheduled for corrective surgery.
Orthopedic problems make up a significant proportion of cases evaluated on the dual-slice SPECT/CT system at the University of Erlangen in Germany. Clearly, no one would recommend that every patient with low back pain receive a SPECT/CT examination, said Prof. Torsten Kuwert, chair of nuclear medicine. But in certain cases, the hybrid scanner can resolve difficult differential diagnoses.
Kuwert is especially interested in the use of SPECT/CT to distinguish degenerative skeletal disease from bone metastases. An investigation is currently under way to assess the value of SPECT/CT in clarifying ambiguous findings from bone scans. Cancer patients undergoing whole-body planar scintigraphy with Tc-99m-labeled diphosphonates would traditionally have been scheduled for additional tests, including MRI, if the radiographs proved inconclusive.
Access to the hybrid scanner should permit necessary follow-up to be performed while the patient is still in situ, while also improving diagnostic accuracy. Early results from a group of 40 cancer patients suggest that the combined imaging approach does indeed yield considerable gain in specificity. Out of 31 lesions identified by enhanced tracer uptake on planar imaging, CT showed 11 to be osteolyses. A further 16 hot spots could be linked to degenerative changes, while only four remained ambiguous and warranted additional follow-up.2
EMERGING APPLICATIONS
Evaluation of SPECT/CT's clinical value has just begun. One niche application for SPECT/CT could be the evaluation of patients with renal colic, according to Ziffer. The warm climate and hard water in south Florida send more than the usual number of patients with kidney stones to Miami's Baptist Hospital. The standard workup for renal colic is an unenhanced CT scan of the abdomen and pelvis. While this is usually sufficient for diagnosis, it can sometimes be difficult to differentiate phleboleths from renal stones.
It can also be hard to glean prognostic information from the unenhanced scans. Administration of radiographic contrast to provide information on the degree of obstruction could easily mask a calcified stone, Ziffer said. He instead plans to administer Tc-99m mercaptoacetylglycine (MAG3) and the diuretic furosemide, then use SPECT/CT to gain an overview of kinetic and anatomic information.
"We do around 10 to 20 CT scans every day for renal colic," he said. "When patients come in, doctors want to know whether there is a stone, where it is, how big it is, and the degree of obstruction. With SPECT/CT, we could do this in one very quick study."
A number of other sites are investigating how SPECT/CT could improve applications where quantification is desirable. At M.D. Anderson, for example, researchers are using the dual modality to look at the distribution of therapeutic tracers. Candidates for investigation include I-131 for thyroid cancer and two treatments for non-Hodgkin's lymphoma: the monoclonal antibody Zevalin (administered first with In-111 and then yttrium-90) and Bexxar (administered as tositumomab and I-131 tositumomab).
Mahood is hopeful that SPECT/CT dosimetry studies may eventually lead to a more accurate measure of radiation delivery. Figures used currently in clinical practice are based on assumptions of organ mass. A low-dose CT scan should provide a better estimation of mass, volume, and radioisotope distribution, he said.
Kuwert is also subjecting the hybrid system to a trial for therapeutic oncology applications. Patients scheduled for Y-90 therapy for hepatocellular carcinoma are first injected with Tc-99m-labeled albumin to assess vascularity. CT data are acquired at the same time to determine the mass and volume of the liver and gain additional anatomic information on the target tumor.
"Before we got the SPECT/CT, we did this manually, and it was a very laborious process. It has become much easier to do the therapy planning with this system," he said.
Another area where SPECT/CT may aid quantification of functional information is ventilation-perfusion scanning. A combined imaging examination that includes a SPECT perfusion scan and also CT pulmonary angiography should provide a better assessment of residual lung function in acute patients with suspected pulmonary embolism, Mahmood said.
"If you do CT pulmonary angiography on its own, it is sometimes very difficult to see peripheral defects. But if you combine it with perfusion, it becomes easy to pick up where there is a perfusion defect," he said. n
Ms. Gould is a contributing editor of Diagnostic Imaging.
http://en.wikipedia.org/wiki/SPECT
Single photon emission computed tomography
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Single photon emission computed tomography (SPECT, or less commonly, SPET) is a nuclear medicine tomographic imaging technique using gamma rays. It is very similar to conventional nuclear medicine planar imaging using a gamma camera. However, it is able to provide true 3D information. This information is typically presented as cross-sectional slices through the patient, but can be freely reformatted or manipulated as required.
A Lung SPECT / CT Fusion image
Contents
[hide]
1 Principles
2 Application
2.1 Myocardial perfusion imaging
2.2 Functional brain imaging
3 Reconstruction
4 Further reading
5 Typical SPECT acquisition protocols
6 See also
7 External links
//
[edit] Principles
In the same way that a plain X-ray is a 2-dimensional (2-D) view of a 3-dimensional structure, the image obtained by a gamma camera is a 2-D view of 3-D distribution of a radionuclide.
SPECT imaging is performed by using a gamma camera to acquire multiple 2-D images (also called projections), from multiple angles. A computer is then used to apply a tomographic reconstruction algorithm to the multiple projections, yielding a 3-D dataset. This dataset may then be manipulated to show thin slices along any chosen axis of the body, similar to those obtained from other tomographic techniques, such as MRI, CT, and PET.
Because SPECT acquisition is very similar to planar gamma camera imaging, the same radiopharmaceuticals may be used. If a patient is examined in another type of nuclear medicine scan but the images are non-diagnostic, it may be possible to proceed straight to SPECT by moving the patient to a SPECT instrument, or even by simply reconfiguring the camera for SPECT image acquisition while the patient remains on the table.
To acquire SPECT images, the gamma camera is rotated around the patient. Projections are acquired at defined points during the rotation, typically every 3-6 degrees. In most cases, a full 360 degree rotation is used to obtain an optimal reconstruction. The time taken to obtain each projection is also variable, but 15 – 20 seconds is typical. This gives a total scan time of 15-20 minutes.
Multi-headed gamma cameras can provide accelerated acquisition. For example, a dual headed camera can be used with heads spaced 180 degrees apart, allowing 2 projections to be acquired simultaneously, with each head requiring 180 degrees of rotation. Triple-head cameras with 120 degree spacing are also used.
Cardiac gated acquisitions are possible with SPECT, just as with planar imaging techniques such as MUGA. Triggered by Electrocardiogram (EKG) to obtain differential information about the heart in various parts of its cycle, gated myocardial SPECT can be used to obtain quantitative information about myocardial perfusion, thickness, and contractility of the myocardium during various parts of the cardiac cycle; and also to allow calculation of left ventricular ejection fraction, stroke volume, and cardiac output.
[edit] Application
SPECT can be used to complement any gamma imaging study, where a true 3D representation can be helpful. E.g. tumor imaging, infection (leukocyte) imaging, thyroid imaging or bone imaging.
Because SPECT permits accurate localisation in 3D space, it can be used to provide information about localised function in internal organs. E.g. functional cardiac or brain imaging.
[edit] Myocardial perfusion imaging
Myocardial perfusion imaging (MPI) is a form of functional cardiac imaging, used for the diagnosis of ischemic heart disease. The underlying principle is that under conditions of stress, diseased myocardium receives less blood flow than normal myocardium. MPI is one of several types of cardiac stress test.
A cardiac specific radiopharmaceutical is administered. E.g. 99mTc-tetrofosmin (Myoview, GE healthcare), 99mTc-sestamibi (Cardiolite, Bristol-Myers Squibb). Following this, the heart rate is raised to induce myocardial stress, either by exercise or pharmacologically with adenosine, dobutamine or dipyridamole (aminophylline can be used to reverse the effects of dipyridamole).
SPECT imaging performed after stress reveals the distribution of the radiopharmaceutical, and therefore the relative blood flow to the different regions of the myocardium. Diagnosis is made by comparing stress images to a further set of images obtained at rest. As the radionuclide redistributes slowly, it is not usually possible to perform both sets of images on the same day, hence a second attendance is required 1-7 days later (although, with a Tl-201 myocardial perfusion study with dipyridamole, rest images can be acquired as little as two-hours post stress). However, if stress imaging is normal, it is unnecessary to perform rest imaging, as it too will be normal – thus stress imaging is normally performed first.
MPI has been demonstrated to have an overall accuracy of about 83% (sensitivity: 85%; specificity: 72%) [1], and is comparable (or better) than other non-invasive tests for ischemic heart disease, including stress echocardiography.
[edit] Functional brain imaging
Usually the gamma-emitting tracer used in functional brain imaging is 99mTc-HMPAO (hexamethylpropylene amine oxime). 99mTc is a metastable nuclear isomer which emits gamma rays which can be detected by a gamma camera. When it is attached to HMPAO, this allows 99mTc to be taken up by brain tissue in a manner proportial to brain blood flow, in turn allowing brain blood flow to be assessed with the nuclear gamma camera.
Because blood flow in the brain is tightly coupled to local brain metabolism and energy use, the 99mTc-HMPAO tracer (as well as the similar 99mTc-EC tracer) is used to assess brain metabolism regionally, in an attempt to diagnose and differentiate the different causal pathologies of dementia. Meta analysis of many reported studies suggests that SPECT with this tracer is about 74% sensitive at diagnosing Alzheimer's disease, vs. 81% sensitivity for clinical exam (mental testing, etc.). More recent studies have show accuracy of SPECT in Alzheimer diagnosis as high as 88% PMID 16785801. In meta analysis, SPECT was superior to clinical exam and clinical criteria (91% vs. 70%) in being able to differentiate Alzheimer's disease from vascular dementias. PMID 15545324 This latter ability relates to SPECT's imaging of local metabolism of the brain, in which the patchy loss of cortical metabolism seen in multiple strokes differs clearly from the more even or "smooth" loss of non-occipital cortical brain function typical of Alzheimer's disease.
99mTc-HMPAO SPECT scanning competes with FDG PET scanning of the brain, which works to assess regional brain glucose metabolism, to provide very similar information about local brain damage from many processes. SPECT is more widely available, however, for the basic reason that the radioisotope generation technology is longer-lasting and far less expensive in SPECT, and the gamma scanning equipment is less expensive as well. The reason for this is that 99mTc (technetium-99m) is extracted from relatively simple technetium-99m generators which are delivered to hospitals and scanning centers weekly, to supply fresh radioisotope, whereas FDG PET relies on FDG which must be made in an expensive medical cyclotron and "hot-lab" (automated chemistry lab for radiopharmaceutical manufacture), then must be delivered directly to scanning sites, with delivery-fraction for each trip handicapped by its natural short 110 minute half-life.
[edit] Reconstruction
Reconstructed images typically have resolutions of 64x64 or 128x128 pixels, with the pixel sizes ranging from 3-6 mm. The number of projections acquired is chosen to be approximately equal to the width of the resulting images. In general, the resulting reconstructed images will be of lower resolution, have increased noise than planar images, and be susceptible to artifacts.
Scanning is time consuming, and it is essential that there is no patient movement during the scan time. Movement can cause significant degradation of the reconstructed images, although movement compensation reconstruction techniques can help with this. A highly uneven distribution of radiopharmaceutical also has the potential to cause artifacts. A very intense area of activity (e.g. the bladder) can cause extensive streaking of the images and obscure neighboring areas of activity. (This is a limitation of the filtered back projection reconstruction algorithm. Iterative reconstruction is an alternative algorithm which is growing in importance, as it is less sensitive to artifacts and can also correct for attenuation and depth dependent blurring).
Attenuation of the gamma rays within the patient can lead to significant underestimation of activity in deep tissues, compared to superficial tissues. Approximate correction is possible, based on relative position of the activity. However, optimal correction is obtained with measured attenuation values. Modern SPECT equipment is available with an integrated x-ray CT scanner. As X-ray CT images are an attenuation map of the tissues, this data can be incorporated into the SPECT reconstruction to correct for attenuation. It also provides a precisely registered CT image which can provide additional anatomical information.
[edit] Further reading
Elhendy et al., Dobutamine Stress Myocardial Perfusion Imaging in Coronary Artery Disease, J Nucl Med 2002 43: 1634-1646
For anyone interested in the brain-imaging applications of SPECT then this is a great review, although the full-text article is not available online without a subscription to the journal. The following link provides a link to the abstract only, and you might be able to access the full article through a membership with a medical library. W. Gordon Frankle, Mark Slifstein, Peter S. Talbot, and Marc Laruelle (2005). "Neuroreceptor Imaging in Psychiatry: Theory and Applications". International Review of Neurobiology, 67: 385-440.
[edit] Typical SPECT acquisition protocols
Study
Radioisotope
Emission energy (keV)
Half-life
Radiopharmaceutical
Activity (MBq)
Rotation (degrees)
Projections
Image resolution
Time per projection (s)
Bone scan
Technetium-99m
140
6 hours
Phosphonates / Bisphosphonates
800
360
120
128 x 128
-
Myocardial perfusion scan
Technetium-99m
140
6 hours
tetrofosmin; Sestamibi
700
180
60
128 x 128
30
Brain scan
Technetium-99m
140
6 hours
HMPAO; ECD
555-1110
360
64
128 x 128
30
Tumor scan
Iodine-123
159
13 hours
MIBG
400
360
60
64 x 64
30
White cell scan
Indium-111 & Technetium-99M
171 & 245
67 hours
in vitro labelled leucocytes
18
360
60
64 x 64
30
[edit] See also
Gamma camera
Neuroimaging
Functional neuroimaging
Magnetic resonance imaging
Positron emission tomography
ISAS (Ictal-Interictal SPECT Analysis by SPM)
http://www.dimag.com/techfocus/SPECT_CT/2006jun/
SPECT/CT settles into variety of clinical practice settings
Contributions to cardiology, oncology, and infection imaging continue, while potential applications emerge from unexpected directions
By: Paula Gould
SPECT/CT has been tagged, rather unkindly, as a modality solution looking for a problem. But as early adopters are showing, it has considerable potential as an all-rounder in busy nuclear medicine departments.
Nuclear cardiology is a prime example. All SPECT images are subject to a certain degree of spatial distortion, caused by differing scatter and absorption of emitted photons before they reach the detector. Correction for this likely attenuation is especially important in cardiac SPECT, where soft-tissue attenuation artifacts can become confused with perfusion defects. Such artifacts can also mask signs of coronary artery disease.
Attenuation correction can be performed relatively easily on hybrid scanners, using anatomic maps derived from CT. The results are impressive, according to Dr. Robert Iwanochko, an assistant professor of medicine at Toronto Western Hospital, who started using SPECT/CT in the hospital's cardiac center this spring.
"We look at all the information, both corrected and uncorrected, so we can see a clear advantage using CT for attenuation correction," he said. "Transmission-based correction is dying. That technology will probably not even be available in two or three more years."
Approximately 30 patients a day undergo SPECT at Toronto Western's cardiac center. One-third of these will be directed onto the hybrid scanner. Patients with a high body mass index are usually directed to the SPECT/CT system, given that attenuation artifacts are theoretically worse in obese subjects.
Iwanochko is keen to test how SPECT/CT fares against a top-class rival. The gold standard for attenuation correction in perfusion imaging at present is rubidium-82 PET, he said. A head-to-head comparison between the two techniques is likely once the planned installation of a new PET scanner at the cardiac center has been completed.
For now, however, Iwanochko is concentrating on building experience with SPECT/CT. Time will tell whether the hybrid scanner lives up to its promise of improving diagnostic confidence.
"The 'equivocal/probable' category tends to shrink when you have good attenuation correction, and that's what we are hoping for," he said. "Patients are either assigned a defect on the basis of SPECT imaging, or they are assigned to be normal."
The 16-slice SPECT/CT system up and running since July 2005 at Baptist Hospital in Miami was purchased primarily with cardiac applications in mind. Approximately 60% of the workload is now cardiac SPECT/CT, according to Dr. Jack Ziffer, chief of radiology. A second system is due to come online once room renovations are complete, which will ease demands for time on the hybrid scanner.
Attenuation correction for myocardial perfusion imaging has proven to be robust, Ziffer said. Prone scans are now unnecessary in nearly all cases, leading to faster examinations. Diagnostic-quality CT also permits coronary calcium scoring to be carried out while patients are still on the table. If calcium scores indicate a low risk of myocardial infarction, patients may then be triaged home without the need for stress testing.
Achieving a good match between the CT and SPECT images was not automatic, Ziffer said. CT data for attenuation correction had previously been acquired on a nondiagnostic scanner during respiration. The images may have been blurred, but at least they fit well with the cardiac SPECT data, which were also acquired during respiration. Moving to 16-slice CT raised the question of exactly when to image: during slow breathing, inspiration, or expiration?
"We have learned that end-tidal expiration is the best time to do the CT," Ziffer said. "We still use a software application to align the heart with the mediastinum, because sometimes the nuclear and CT images are not exact. That's an important piece for getting good cardiac attenuation correction."
The six-slice SPECT/CT system acquired by The Cleveland Clinic Foundation is similarly being used for both attenuation correction and calcium scoring during cardiac examinations. Time for cardiac studies is limited, however, given competing demands on the system from other specialties, said Dr. Donald Neumann, director of molecular oncologic applications. The real advantages of hybrid imaging are crucial when it comes to scheduling.
"We have tended to focus on larger patients, with the hope of removing some of the problems associated with attenuation artifacts," he said. "But it is premature at this point to say which cardiac patient is going to benefit most from that unit."
LOCALIZING PATHOLOGY
Many neurologic applications are also likely to reap benefits from SPECT/CT through superior attenuation correction, according to Neumann. Where oncology can gain is through anatomic localization.
Nuclear medicine techniques are adept at seeking out areas of malignancy. The growing armamentarium of radioisotope tracers is also widening options for cancer diagnosis, staging, and follow-up far beyond FDG-PET. The poor spatial resolution of nuclear medicine images can make it difficult to pinpoint the exact location of pathology, however, with important implications for therapy planning and the use of imaging to monitor treatment efficacy.
Overlaying PET or SPECT images with CT data can provide the ideal combination of functional and anatomic information in a single view. If both sets of data have been collected in the same examination, making a good match is likely to be much easier.
Patients undergoing indium-111 octreotide studies, meta-iodobenzylguanidine (MIBG) studies for neuroblastoma, and In-111 Prostascint for prostate antibody imaging are all routinely scheduled onto the Cleveland Clinic's SPECT/CT unit. Cases of hyperparathyroid tumors and suspected hepatic hemangiomas are evaluated on the hybrid system as well.
"On occasion, we also get a call for evaluation of an abdominal mass, with the suspicion of a possible accessory spleen or splenic remnant, and those will definitely be performed on the SPECT/CT, too," Neumann said.
The improved capacity for anatomic localization has yielded additional benefits in lymphoscintigraphy.1 The lymphatic drainage pattern is often ambiguous in patients with head and/or neck melanomas, he said. Surgeons looking for help in localizing the sentinel lymph nodes have found that coregistered CT and SPECT images provide the information they require.
"I used to be able to say 'I think there's a sentinel lymph node in the right neck.' Now I can tell 'There are three sentinel lymph nodes: One is at right level Ib, the second one is at level Va, and the third is at level III on the right," Neumann said.
Sentinel lymph node mapping is also much quicker when using a single SPECT/CT unit, according to Dr. Shahid Mahmood, clinical director of Mount Elizabeth Hospital in Singapore. Mount Elizabeth's nuclear medicine and PET center receives patients from Malaysia, Indonesia, and Thailand as well as from Singapore itself. The new six-slice SPECT/CT unit, which came online in September 2005, is also being used in diagnostic oncology to improve the throughput and accuracy of octreotide studies and parathyroid imaging.
Dr. Homer Macapinlac, chair of nuclear medicine at M.D. Anderson Cancer Center in Houston, was so confident that SPECT/CT would benefit the center's oncology practice that he supported investment in five new multislice hybrid scanners. All systems came online together for clinical work at the start of this year. Now physicians are working to gain experience with the protocols they have instituted.
One prime area of use is likely to be screening for bone metastases in patients with breast or prostate cancer. Nuclear medicine bone scans, on their own, can often suggest that patients are getting worse, Macapinlac said. So-called flare phenomena can be misinterpreted as metastatic progression when, in fact, patients are responding to treatment. Performing CT with SPECT could confirm this by showing whether lesions are becoming more calcified. This finding would then remove the need for a follow-up bone scan in three to six months and reduce unnecessary patient anxiety.
"This could make us more efficient and effective, in diminishing the number of times we scan to try and evaluate whether these patients are getting better or worse," Macapinlac said. "Having CT together with SPECT should make this a stronger modality for evaluating metastatic disease."
Other diagnostic oncology applications at M.D. Anderson include In-111 octreotide for neuroendocrine tumors, I-123 MIBG imaging of neuroblastomas in pediatric patients, and presurgical localization of parathyroid adenomas using technetium-99m 2-methoxy isobutyl isonitrile (MIBI).
"I now ask myself why we never had these machines before," Macapinlac said. "The additive information from the SPECT and CT together make them potentially more powerful and accurate than the separate studies. SPECT contributes to the interpretation of the CT study, and vice versa."
Another issue physicians must grapple with is how to deal with patients whose scans may have been misdiagnosed in the past, Macapinlac said. Previous reports based on separate SPECT and CT scans may have failed to identify true malignancies and/or benign lesions because findings appeared ambiguous.
"With the better imaging we now have, we are trying to update the status of our patients in the best way that we can," he said. "It's not just identification of disease but also the identification of what is benign. That can be more important than the identification of the cancer itself."
Most SPECT/CT studies performed at Maasland Hospital in Sittard, the Netherlands, are likewise related to oncology applications. The hybrid system, which has an integrated dual-slice CT scanner, came online for clinical work in August 2005.
All patients suspected of having bone metastases are now scheduled for a SPECT/CT scan. This helps differentiate true metastases from, for instance, signs of trauma or degenerative joint disease, said Dr. Paul Thimister, a nuclear physician at Maasland. Meanwhile, sentinel lymph node imaging for breast cancer patients and imaging of patients with parathyroid tumors are proving to be of considerable use to the surgical team.
"We have seen some very small parathyroid tumors, and with the combined SPECT/CT, we were able to locate the node exactly. We had two patients in which this was in the mediastinal region, so it was very important for the surgeon to know whether he had to perform a sternotomy to reach the node," Thimister said.
Upgrading to SPECT/CT has made presurgical assessment of parathyroid adenomas a great deal easier, according to Ziffer. Prior to installation of the hybrid scanner, patients first underwent planar imaging and SPECT with Tc-99m MIBI. Adorned with appropriately placed radiopaque markers, they would then be moved to a separate unit for neck CT.
The strategy provided surgeons with sufficient anatomic detail that they could perform accurate parathyroid adenoma surgery extremely quickly, Ziffer said. But nuclear physicians and patients had to cope with a relatively cumbersome procedure. Now all imaging can be performed on a single machine, removing the need for markers and movement between scanners.
"This has streamlined things and made image registration much more exact," Ziffer said.
At New York City's Lenox Hill Hospital, oncology applications have taken a back seat to other uses. Here the year-old 16-slice SPECT/CT system is more often used to seek out signs of infection, using the radioisotopes In-111 and gallium-67. As with oncology, infection imaging is another area of nuclear medicine that can benefit from the greater anatomic localization provided by SPECT/CT. More accurate identification of infection sites can assist considerably with treatment planning.
ORTHOPEDIC OPTIONS
The biggest impact of SPECT/CT at Lenox Hill, however, has been in orthopedics, according to Dr. Stephen Scharf, chief of nuclear medicine. Scharf is using the hybrid system to improve presurgical evaluation of the large number of patients referred to nuclear medicine with chronic foot pain. Surgery is generally regarded as a last resort for these patients. Bone scans are typically performed prior to intervention, so surgeons can double-check that their planned strategy is targeting the correct area.
"When we moved to doing this patient group on SPECT/CT, we found that the detail we could give the surgeons was in some cases sufficient to change their mind about whether to do surgery," Scharf said. "It gives us tremendous anatomic detail and allows us to characterize pathology in a way that we have never been able to do before."
A considerable number of arthritic patients with chronic pain in the arches of the foot, or the midfoot, will benefit from drastic surgical intervention such as bone fusion, Scharf said. But intervention may not be the best option for patients whose pain is actually due to an occult fracture or past traumatic episode. SPECT/CT helps differentiate between these two patient groups by distinguishing focal activity along a joint line from activity within the bone itself.
SPECT/CT changed the management of one of the first foot pain patients scanned with it, he said. Imaging sowed sufficient doubts about the wisdom of bone fusion that the surgeon opted instead to administer a local anesthetic and steroids under fluoroscopy guidance. One year later, the patient has yet to experience any recurrence of pain.
The combination of functional and anatomic data may also aid assessment of patients scheduled for spinal surgery, Scharf said. Experience from a handful of patients scheduled for vertebroplasty has highlighted the significance of seeing anatomy in more detail. Vertebroplasty can be an extremely effective way of alleviating pain caused by disc degeneration. But surgeons are unlikely to cement all abnormal discs lest procedures become too long, too expensive, and subject to greater risks. Morphing CT and SPECT data can indicate exactly where the needle should be placed.
"We had one dramatic case in which it was clear that if we had left the surgeon to his own devices, he would have chosen the wrong one," Scharf said. "The abnormality on the bone scan was right at the junction of two vertebrae. The one that was collapsed turned out not to be the one that was abnormal."
He acknowledges that orthopedic imaging is somewhat of a bonus area for SPECT/CT. Little if anything had been said on this topic when the hybrid systems were launched. One reason is likely to be the rise of MRI for musculoskeletal imaging, especially in the U.S. In the face of such competition, many orthopedists have abandoned bone scanning for all but a few specialized areas, Scharf said.
"Around here, I've got 10 MR scanners within 10 blocks," he said. "But now the foot surgeons have started to come back and look at this as an interesting technology, and we are starting to do all of our vertebroplasty patients on SPECT/CT."
Dr. Paul Shreve, a radiologist with PET Medical Imaging Center and Advanced Radiology Services in Grand Rapids, MI, has similarly identified vertebroplasty as an important application for SPECT/CT. He hopes to start working with a hybrid scanner later this year.
"Sometimes it is hard to figure out which vertebral body is hot on the bone scan, and what it corresponds to on the CT scan. With SPECT/CT, we will be able do both examinations at once, and everything will be aligned correctly," he said.
In Europe, where MRI has yet to exert as strong a grip on musculoskeletal imaging, early adopters of SPECT/CT have been quick to spot potential benefits in this area. The first patient imaged with SPECT/CT at Maasland Hospital demonstrated the potential of hybrid scanning to orthopedic applications. The patient was suffering from low back pain, and after bone scintigraphy, clinicians suspected osteomyelitis. But CT data from the region of interest identified on SPECT revealed a fracture in the lower spine and spinal compression. Instead of receiving antibiotics, as had been planned, the patient was scheduled for corrective surgery.
Orthopedic problems make up a significant proportion of cases evaluated on the dual-slice SPECT/CT system at the University of Erlangen in Germany. Clearly, no one would recommend that every patient with low back pain receive a SPECT/CT examination, said Prof. Torsten Kuwert, chair of nuclear medicine. But in certain cases, the hybrid scanner can resolve difficult differential diagnoses.
Kuwert is especially interested in the use of SPECT/CT to distinguish degenerative skeletal disease from bone metastases. An investigation is currently under way to assess the value of SPECT/CT in clarifying ambiguous findings from bone scans. Cancer patients undergoing whole-body planar scintigraphy with Tc-99m-labeled diphosphonates would traditionally have been scheduled for additional tests, including MRI, if the radiographs proved inconclusive.
Access to the hybrid scanner should permit necessary follow-up to be performed while the patient is still in situ, while also improving diagnostic accuracy. Early results from a group of 40 cancer patients suggest that the combined imaging approach does indeed yield considerable gain in specificity. Out of 31 lesions identified by enhanced tracer uptake on planar imaging, CT showed 11 to be osteolyses. A further 16 hot spots could be linked to degenerative changes, while only four remained ambiguous and warranted additional follow-up.2
EMERGING APPLICATIONS
Evaluation of SPECT/CT's clinical value has just begun. One niche application for SPECT/CT could be the evaluation of patients with renal colic, according to Ziffer. The warm climate and hard water in south Florida send more than the usual number of patients with kidney stones to Miami's Baptist Hospital. The standard workup for renal colic is an unenhanced CT scan of the abdomen and pelvis. While this is usually sufficient for diagnosis, it can sometimes be difficult to differentiate phleboleths from renal stones.
It can also be hard to glean prognostic information from the unenhanced scans. Administration of radiographic contrast to provide information on the degree of obstruction could easily mask a calcified stone, Ziffer said. He instead plans to administer Tc-99m mercaptoacetylglycine (MAG3) and the diuretic furosemide, then use SPECT/CT to gain an overview of kinetic and anatomic information.
"We do around 10 to 20 CT scans every day for renal colic," he said. "When patients come in, doctors want to know whether there is a stone, where it is, how big it is, and the degree of obstruction. With SPECT/CT, we could do this in one very quick study."
A number of other sites are investigating how SPECT/CT could improve applications where quantification is desirable. At M.D. Anderson, for example, researchers are using the dual modality to look at the distribution of therapeutic tracers. Candidates for investigation include I-131 for thyroid cancer and two treatments for non-Hodgkin's lymphoma: the monoclonal antibody Zevalin (administered first with In-111 and then yttrium-90) and Bexxar (administered as tositumomab and I-131 tositumomab).
Mahood is hopeful that SPECT/CT dosimetry studies may eventually lead to a more accurate measure of radiation delivery. Figures used currently in clinical practice are based on assumptions of organ mass. A low-dose CT scan should provide a better estimation of mass, volume, and radioisotope distribution, he said.
Kuwert is also subjecting the hybrid system to a trial for therapeutic oncology applications. Patients scheduled for Y-90 therapy for hepatocellular carcinoma are first injected with Tc-99m-labeled albumin to assess vascularity. CT data are acquired at the same time to determine the mass and volume of the liver and gain additional anatomic information on the target tumor.
"Before we got the SPECT/CT, we did this manually, and it was a very laborious process. It has become much easier to do the therapy planning with this system," he said.
Another area where SPECT/CT may aid quantification of functional information is ventilation-perfusion scanning. A combined imaging examination that includes a SPECT perfusion scan and also CT pulmonary angiography should provide a better assessment of residual lung function in acute patients with suspected pulmonary embolism, Mahmood said.
"If you do CT pulmonary angiography on its own, it is sometimes very difficult to see peripheral defects. But if you combine it with perfusion, it becomes easy to pick up where there is a perfusion defect," he said. n
Ms. Gould is a contributing editor of Diagnostic Imaging.
Introduction : PET CT
PET CT
http://en.wikipedia.org/wiki/PET-CT
Positron emission tomography - computed tomography (better know by its acronym PET-CT) is a medical imaging device which combines in a single gantry system both a Positron Emission Tomography (PET) and an x-ray Computed Tomography, so that images acquired from both devices can be taken sequentially, in the same session from the patient and combined into a single superposed (co-registered) image. Thus, functional imaging obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning. Two- and three-dimensional image reconstruction may be rendered as a function of a common software and control system.
PET-CT has revolutionized many fields of medical diagnosis, by adding precision of anatomic localization to functional imaging, which was previously lacking from pure PET imaging. For example, in oncology, surgical planning, radiation therapy and cancer staging have been changing rapidly under the influence of PET-CT availability, to the extent that many diagnostic imaging procedures and centers have been gradually abandoning conventional PET devices and substituting them by PET-CTs. Although the combined device is considerably more expensive, it has the advantage of providing both functions as stand-alone examinations, being, in fact, two devices in one.
The only other obstacle to a wider dissemination of PET-CT is the difficulty and cost of producing and transporting the radiopharmaceuticals used for PET imaging, which are usually extremely short-lived (for instance, the half life of radioactive fluor18 used to trace glucose metabolism (using fluorodeoxyglucose -- FDG) is two hours only. Its production requires a very expensive synchrotron as well as a production line for the radiopharmaceuticals.
[edit] Procedure for FDG imaging
An example of how PET-CT works in the diagnostic work-up of FDG metabolic mapping follows:
Before the exam, the patient undergoes a minimum of 8-hour fasting and rest;
In the day of the exam, the patient rests lying for a minimum of 15 min, in order to quiet down muscular activity, which might be interpreted as abnormal metabolism;
An intravenous bolus injection of a dose of recently produced 2-FDG or 3-FDG is made, usually by a vein in one of the arms. Dosage ranges from 0,1 to 0,2 mCi per kg of body weight;
After one or two hours, the patient is placed into the PET-CT device, usually lying in a supine position with his/her arms resting at the sides, or brought together above the head, depending on the main region of interest (ROI)
An automatic bed moves head first into the gantry, first obtaining a topogram, also called a scout view, which is a kind of whole body flat sagital section, obtained with the X-ray tube fixed into the upper position.
The operator uses the PET-CT computer console to identify the patient and examination, delimit the caudal and rostral limits of the body scan onto the scout view, selects the scanning parameters and starts the image acquisition period, which follows without human intervention;
The patient is automatically moved head first into the CT gantry, and the x-ray tomogram is acquired;
Now the patient is automatically moved through the PET gantry, which is mounted in parallel with the CT gantry, and the PET slices are acquired;
The patient may now leave the device, and the PET-CT software starts reconstructing and aligning the PET and CT images.
A whole body scan, which usually is made from mid-thights to the top of the head, takes about 40 min. FDG imaging protocols acquires slices with a thickness of 2 to 3 mm. Hypermetabolic lesions are shown as false color-coded pixels or voxels onto the gray-value coded CT images. Standard Uptake Values are calculated by the software for each hypermetabolic region detected in the image. It provides a quantification of size of the lesion, since functional imaging does not provide a precise anatomical estimate of its extent. The CT can be used for that, when the lesion is also visualized in its images (this is not always the case when hypermetabolic lesions are not accompanied by anatomical changes).
FDG doses in quantities sufficient to carry out 4-5 examinations are delivered daily, twice or more times per day, by the provider to the diagnostic imaging center.
For uses in stereotactic radiation therapy of cancer, special fiducial marks are placed in the patient's body before acquiring the PET-CT images. The slices thus acquired may be transferred digitally to a linear accelerator which is used to perform precise bombardment of the target areas using high energy photons (radiosurgery).
PET
http://en.wikipedia.org/wiki/Positron_emission_tomography
Positron emission tomography
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Image of a typical positron emission tomography (PET) facility
Positron emission tomography (PET) is a nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule. Images of tracer concentration in 3-dimensional space within the body are then reconstructed by computer analysis. In modern scanners, this reconstruction is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine.
If the biologically active molecule chosen for PET is FDG, an analogue of glucose, the concentrations of tracer imaged then give tissue metabolic activity, in terms of regional glucose uptake. Although use of this tracer results in the most common type of PET scan, other tracer molecules are used in PET to image the tissue concentration of many other types of molecules of interest.
Contents
[hide]
1 Description
1.1 Operation
1.2 Localization of the positron annihilation event
1.3 Image reconstruction using coincidence statistics
1.4 Combination of PET with CT and MRI
1.5 Radioisotopes
1.6 Limitations
1.7 Image reconstruction
2 History
3 Applications
4 Safety
5 See also
6 References
7 Further reading
8 External links
//
[edit] Description
Schematic view of a detector block and ring of a PET scanner
[edit] Operation
To conduct the scan, a short-lived radioactive tracer isotope, which decays by emitting a positron, which also has been chemically incorporated into a biologically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the active molecule becomes concentrated in tissues of interest; then the research subject or patient is placed in the imaging scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour.
Schema of a PET acquisition process
As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, the antimatter counterpart of an electron. After travelling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of annihilation (gamma) photons moving in opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes or silicon avalanche photodiodes (Si APD). The technique depends on simultaneous or coincident detection of the pair of photons; photons which do not arrive in pairs (i.e. within a few nanoseconds) are ignored.
[edit] Localization of the positron annihilation event
The most significant fraction of electron-positron decays result in two 511 keV gamma photons being emitted at almost 180 degrees to each other; hence it is possible to localize their source along a straight line of coincidence (also called formally the line of response or LOR). In practice the LOR has a finite width as the emitted photons are not exactly 180 degrees apart. If the recovery time of detectors is in the picosecond range rather than the 10's of nanosecond range, it is possible to calculate the single point on the LOR at which an annihilation event originated, by measuring the "time of flight" of the two photons. This technology is not yet common, but it is available on some new systems [1].
[edit] Image reconstruction using coincidence statistics
More commonly, a technique much like the reconstruction of computed tomography (CT) and single photon emission computed tomography (SPECT) data is used, although the data set collected in PET is much poorer than CT, so reconstruction techniques are more difficult (see Image reconstruction of PET).
Using statistics collected from tens-of-thousands of coincidence events, a set of simultaneous equations for the total activity of each parcel of tissue along many LORs can be solved by a number of techniques, and thus a map of radioactivities as a function of location for parcels or bits of tissue (also called voxels), may be constructed and plotted. The resulting map shows the tissues in which the molecular probe has become concentrated, and can be interpreted by a nuclear medicine physician or radiologist in the context of the patient's diagnosis and treatment plan.
A complete body PET / CT Fusion image
A Brain PET / MRI Fusion image
[edit] Combination of PET with CT and MRI
PET scans are increasingly read alongside CT or magnetic resonance imaging (MRI) scans, the combination ("co-registration") giving both anatomic and metabolic information (i.e., what the structure is, and what it is doing biochemically). Because PET imaging is most useful in combination with anatomical imaging, such as CT, modern PET scanners are now available with integrated high-end multi-detector-row CT scanners. Because the two scans can be performed in immediate sequence during the same session, with the patient not changing position between the two types of scans, the two sets of images are more-precisely registered, so that areas of abnormality on the PET imaging can be more perfectly correlated with anatomy on the CT images. This is very useful in showing detailed views of moving organs or structures with higher amounts of anatomical variation, such as are more likely to occur outside the brain.
[edit] Radioisotopes
Radionuclides used in PET scanning are typically isotopes with short half lives such as carbon-11 (~20 min), nitrogen-13 (~10 min), oxygen-15 (~2 min), and fluorine-18 (~110 min). These radionuclides are incorporated either into compounds normally used by the body such as glucose (or glucose analogues), water or ammonia, or into molecules that bind to receptors or other sites of drug action. Such labelled compounds are known as radiotracers. It is important to recognize that PET technology can be used to trace the biologic pathway of any compound in living humans (and many other species as well), provided it can be radiolabeled with a PET isotope. Thus the specific processes that can be probed with PET are virtually limitless, and radiotracers for new target molecules and processes are being synthesized all the time; as of this writing there are already dozens in clinical use and hundreds applied in research. Due to the short half lives of most radioisotopes, the radiotracers must be produced using a cyclotron and radiochemistry laboratory that are in close proximity to the PET imaging facility. The half life of fluorine-18 is long enough such that fluorine-18 labeled radiotracers can be manufactured commercially at an offsite location.
[edit] Limitations
The minimization of radiation dose to the subject is an attractive feature of the use of short-lived radionuclides. Besides its established role as a diagnostic technique, PET has an expanding role as a method to assess the response to therapy, in particular, cancer therapy,[1] where the risk to the patient from lack of knowledge about disease progress is much greater than the risk from the test radiation.
Limitations to the widespread use of PET arise from the high costs of cyclotrons needed to produce the short-lived radionuclides for PET scanning and the need for specially adapted on-site chemical synthesis apparatus to produce the radiopharmaceuticals. Few hospitals and universities are capable of maintaining such systems, and most clinical PET is supported by third-party suppliers of radiotracers which can supply many sites simultaneously. This limitation restricts clinical PET primarily to the use of tracers labelled with F-18, which has a half life of 110 minutes and can be transported a reasonable distance before use, or to rubidium-82, which can be created in a portable generator and is used for myocardial perfusion studies. Nevertheless, in recent years a few on-site cyclotrons with integrated shielding and hot labs have begun to accompany PET units to remote hospitals. The presence of the small on-site cyclotron promises to expand in the future as the cyclotrons shrink in response to the high cost of isotope transportation to remote PET machines [2]
Because the half-life of F-18 is about two hours, the prepared dose of a radiopharmaceutical bearing this radionuclide will undergo multiple half-lives of decay during the working day. This necessitates frequent recalibration of the remaining dose (determination of activity per unit volume) and careful planning with respect to patient scheduling.
[edit] Image reconstruction
The raw data collected by a PET scanner are a list of 'coincidence events' representing near-simultaneous detection of annihilation photons by a pair of detectors. Each coincidence event represents a line in space connecting the two detectors along which the positron emission occurred.
Coincidence events can be grouped into projections images, called sinograms. The sinograms are sorted by the angle of each view and tilt, the latter in 3D case images. The sinogram images are analogous to the projections captured by computed tomography (CT) scanners, and can be reconstructed in a similar way. However, the statistics of the data is much worse than those obtained through transmission tomography. A normal PET data set has millions of counts for the whole acquisition, while the CT can reach a few billion counts. As such, PET data suffer from scatter and random events much more dramatically than CT data does.
In practice, considerable pre-processing of the data is required - correction for random coincidences, estimation and subtraction of scattered photons, detector dead-time correction (after the detection of a photon, the detector must "cool down" again) and detector-sensitivity correction (for both inherent detector sensitivity and changes in sensitivity due to angle of incidence).
Filtered back projection (FBP) has been frequently used to reconstruct images from the projections. This algorithm has the advantage of being simple while having a low requirement for computing resources. However, shot noise in the raw data is prominent in the reconstructed images and areas of high tracer uptake tend to form streaks across the image.
Iterative expectation-maximization algorithms are now the preferred method of reconstruction. The advantage is a better noise profile and resistance to the streak artifacts common with FBP, but the disadvantage is higher computer resource requirements.
Attenuation correction: As different LORs must traverse different thicknesses of tissue, the photons are attenuated differentially. The result is that structures deep in the body are reconstructed as having falsely low tracer uptake. Contemporary scanners can estimate attenuation using integrated x-ray CT equipment, however earlier equipment offered a crude form of CT using a gamma ray (positron emitting) source and the PET detectors.
While attenuation corrected images are generally more faithful representations, the correction process is itself susceptible to significant artifacts. As a result, both corrected and uncorrected images are always reconstructed and read together.
2D/3D reconstruction: Early PET scanners had only a single ring of detectors, hence the acquisition of data and subsequent reconstruction was restricted to a single transverse plane. More modern scanners now include multiple rings, essentially forming a cylinder of detectors.
There are two approaches to reconstructing data from such a scanner: 1) treat each ring as a separate entity, so that only coincidences within a ring are detected, the image from each ring can then be reconstructed individually (2D reconstruction), or 2) allow coincidences to be detected between rings as well as within rings, then reconstruct the entire volume together (3D).
3D techniques have better sensitivity (because more coincidences are detected and used) and therefore less noise, but are more sensitive to the effects of scatter and random coincidences, as well as requiring correspondingly greater computer resources.
[edit] History
The concept of emission and transmission tomography was introduced by David Kuhl and Roy Edwards in the late 1950s. Their work later led to the design and construction of several tomographic instruments at the University of Pennsylvania. Tomographic imaging techniques were further developed by Michel Ter-Pogossian, Michael E. Phelps and others at the Washington University School of Medicine.[3][4]
In the 1970s, Tatsuo Ido at the Brookhaven National Laboratory was the first to describe the synthesis of 18F-FDG, the most commonly used PET scanning isotope carrier. The compound was first administered to two normal human volunteers by Abass Alavi in August 1976 at the University of Pennsylvania. Brain images obtained with an ordinary (non-PET) nuclear scanner demonstrated the concentration of FDG in that organ. Later, the substance was used in dedicated positron tomographic scanners, to yield the modern procedure.
[edit] Applications
Maximum intensity projection (MIP) of a typical F-18 FDG wholebody PET acquisition
PET is both a medical and research tool. It is used heavily in clinical oncology (medical imaging of tumors and the search for metastases), and for clinical diagnosis of certain diffuse brain diseases such as those causing various types of dementias. PET is also an important research tool to map normal human brain and heart function.
PET is also used in pre-clinical studies using animals, where it allows repeated investigations into the same subjects. This is particularly valuable in cancer research, as it results in an increase in the statistical quality of the data (subjects can act as their own control) and substantially reduces the numbers of animals required for a given study.
Alternative methods of scanning include x-ray computed tomography (CT), magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI), ultrasound and single photon emission computed tomography (SPECT).
While some imaging scans such as CT and MRI isolate organic anatomic changes in the body, PET and SPECT are capable of detecting areas of molecular biology detail (even prior to anatomic change). PET scanning does this using radiolabelled molecular probes that have different rates of uptake depending on the type and function of tissue involved. Changing of regional blood flow in various anatomic structures (as a measure of the injected positron emitter) can be visualized and relatively quantified with a PET scan.
PET imaging is best performed using a dedicated PET scanner. However, it is possible to acquire PET images using a conventional dual-head gamma camera fitted with a coincidence detector. The quality of gamma-camera PET is considerably lower, and acquisition is slower. However, for institutions with low demand for PET, this may allow on-site imaging, instead of referring patients to another center, or relying on a visit by a mobile scanner.
PET is a valuable technique for some diseases and disorders, because it is possible to target the radio-chemicals used for particular bodily functions.
Oncology: PET scanning with the tracer fluorine-18 (F-18) fluorodeoxyglucose (FDG), called FDG-PET, is widely used in clinical oncology. This tracer is a glucose analog that is taken up by glucose-using cells and phosphorylated by hexokinase (whose mitochondrial form is greatly elevated in rapidly-growing malignant tumours). A typical dose of FDG used in an oncological scan is 200-400 MBq for an adult human. Because the oxygen atom which is replaced by F-18 to generate FDG is required for the next step in glucose metabolism in all cells, no further reactions occur in FDG. Furthermore, most tissues (with the notable exception of liver and kidneys) cannot remove the phosphate added by hexokinase. This means that FDG is trapped in any cell which takes it up, until it decays, since phosphorylated sugars, due to their ionic charge, cannot exit from the cell. This results in intense radiolabeling of tissues with high glucose uptake, such as the brain, the liver, and most cancers. As a result, FDG-PET can be used for diagnosis, staging, and monitoring treatment of cancers, particularly in Hodgkin's disease, non Hodgkin's lymphoma, and lung cancer. Many other types of solid tumors will be found to be very highly labeled on a case-by-case basis-- a fact which becomes especially useful in searching for tumor metastasis, or for recurrence after a known highly-active primary tumor is removed. Because individual PET scans are more expensive than "conventional" imaging with computed tomography (CT) and magnetic resonance imaging (MRI), expansion of FDG-PET in cost-constrained health services will depend on proper health technology assessment; this problem is a difficult one because structural and functional imaging often cannot be directly compared, as they provide different information. Oncology scans using FDG make up over 90% of all PET scans in current practice.
PET scan of the human brain.
Neurology: PET neuroimaging is based on an assumption that areas of high radioactivity are associated with brain activity. What is actually measured indirectly is the flow of blood to different parts of the brain, which is generally believed to be correlated, and has been measured using the tracer oxygen-15. However, because of its 2-minute half-life O-15 must be piped directly from a medical cyclotron for such uses, and this is difficult. In practice, since the brain is normally a rapid user of glucose, and since brain pathologies such as Alzheimer's disease greatly decrease brain metabolism of both glucose and oxygen in tandem, standard FDG-PET of the brain, which measures regional glucose use, may also be successfully used to differentiate Alzheimer's disease from other dementing processes, and also to make early diagnosis of Alzheimer's disease. The advantage of FDG-PET for these uses is its much wider availability. PET imaging with FDG can also be used for localization of seizure focus: A seizure focus will appear as hypometabolic during an interictal scan. Several radiotracers (i.e. radioligands) have been developed for PET that are ligands for specific neuroreceptor subtypes such as [11C] raclopride and [18F] fallypride for dopamine D2/D3 receptors, [11C]McN 5652 and [11C]DASB for serotonin transporters, or enzyme substrates (e.g. 6-FDOPA for the AADC enzyme). These agents permit the visualization of neuroreceptor pools in the context of a plurality of neuropsychiatric and neurologic illnesses. A novel probe developed at the University of Pittsburgh termed PIB (Pittsburgh Compound-B) permits the visualization of amyloid plaques in the brains of Alzheimer's patients. This technology could assist clinicians in making a positive clinical diagnosis of AD pre-mortem and aid in the development of novel anti-amyloid therapies.
Cardiology, atherosclerosis and vascular disease study: In clinical cardiology, FDG-PET can identify so-called "hibernating myocardium", but its cost-effectiveness in this role versus SPECT is unclear. Recently, a role has been suggested for FDG-PET imaging of atherosclerosis to detect patients at risk of stroke [2].
Neuropsychology / Cognitive neuroscience: To examine links between specific psychological processes or disorders and brain activity.
Psychiatry: Numerous compounds that bind selectively to neuroreceptors of interest in biological psychiatry have been radiolabeled with C-11 or F-18. Radioligands that bind to dopamine receptors (D1,D2, reuptake transporter), serotonin receptors (5HT1A, 5HT2A, reuptake transporter) opioid receptors (mu) and other sites have been used successfully in studies with human subjects. Studies have been performed examining the state of these receptors in patients compared to healthy controls in schizophrenia, substance abuse, mood disorders and other psychiatric conditions.
Pharmacology: In pre-clinical trials, it is possible to radiolabel a new drug and inject it into animals. The uptake of the drug, the tissues in which it concentrates, and its eventual elimination, can be monitored far more quickly and cost effectively than the older technique of killing and dissecting the animals to discover the same information. PET scanners for rats and non-human primates are marketed for this purpose. The technique is still generally too expensive for the veterinary medicine market, however, so very few pet PET scans are done. Drug occupancy at the purported site of action can also be inferred indirectly by competition studies between unlabeled drug and radiolabeled compounds known apriori to bind with specificity to the site.
[edit] Safety
PET scanning is non-invasive, but it does involve exposure to ionizing radiation. The total dose of radiation is small, however, usually around 7 mSv. This can be compared to 2.2 mSv average annual background radiation in the UK, 0.02 mSv for a chest x-ray, up to 8 mSv for a CT scan of the chest, 2-6 mSv per annum for aircrew (data from UK National Radiological Protection Board). Patients with small children may be advised to limit proximity to them for several hours following the completion of the test.
http://www.radiologyinfo.org/en/info.cfm?pg=pet&bhcp=1
Positron Emission Tomography (PET) Scanning
What is Positron Emission Tomography (PET) Scanning?
What are some common uses of the procedure?
How should I prepare for the procedure?
What does the equipment look like?
How does the procedure work?
How is the procedure performed?
What will I experience during and after procedure?
Who interprets the results and how do I get them?
What are the benefits vs. risks?
What are the limitations of PET?
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What is Positron Emission Tomography (PET) Scanning
Sample image obtained using a combination of PET and CT imaging technology.
Positron emission tomography, also called PET imaging or a PET scan, is a type of nuclear medicine imaging.
Nuclear medicine is a subspecialty within the field of radiology that uses very small amounts of radioactive material to diagnose or treat disease and other abnormalities within the body.
Nuclear medicine imaging procedures are noninvasive and usually painless medical tests that help physicians diagnose medical conditions. These imaging scans use radioactive materials called a radiopharmaceutical or radiotracer.
Depending on the type of nuclear medicine exam you are undergoing, the radiotracer is injected into a vein, swallowed by mouth or inhaled as a gas and eventually collects in the area of your body being examined, where it gives off energy in the form of gamma rays. This energy is detected by a device called a gamma camera, a (positron emission tomography) PET scanner and/or probe. These devices work together with a computer to measure the amount of radiotracer absorbed by your body and to produce special pictures offering details on both the structure and function of organs and other internal body parts.
In some centers, nuclear medicine images can be superimposed with computed tomography (CT) or magnetic resonance imaging (MRI) to produce special views, a practice known as image fusion or co-registration. These views allow the information from two different studies to be correlated and interpreted on one image, leading to more precise information and accurate diagnoses.
A PET scan measures important body functions, such as blood flow, oxygen use, and sugar (glucose) metabolism, to help doctors evaluate how well organs and tissues are functioning.
What are some common uses of the procedure?
PET scans are performed to:
detect cancer
determine how much a cancer has spread in the body
assess the effectiveness of a treatment plan, such as cancer therapy
determine if a cancer has returned after treatment
determine blood flow to the heart muscle
determine the effects of a heart attack, or myocardial infarction, on areas of the heart
identify areas of the heart muscle that would benefit from a procedure such as angioplasty or coronary artery bypass surgery (in combination with a myocardial perfusion scan).
evaluate brain abnormalities, such as tumors, memory disorders and seizures and other central nervous system disorders
to map normal human brain and heart function
How should I prepare?
You may be asked to wear a gown during the exam or you may be allowed to wear your own clothing.
Women should always inform their physician or technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby. See the Safety page for more information about pregnancy and breastfeeding related to nuclear medicine imaging.
You should inform your physician of any medications you are taking as well as vitamins and herbal supplements and if you have any allergies. Also inform your doctor about recent illnesses or other medical conditions.
Jewelry and other accessories should be left at home if possible, or removed prior to the exam because they may interfere with the procedure.
You will receive specific instructions based on the type of PET scan you are undergoing. Diabetic patients will receive special instructions to prepare for this exam.
What does the equipment look like?
A positron emission tomography (PET) scanner is a large machine with a round, doughnut shaped hole in the middle, similar to a CT unit. Within this machine are multiple rings of detectors that record the emission of energy from the radiotracer in your body.
A nearby computer aids in creating the images from the data obtained by the camera or scanner.
How does the procedure work?
With ordinary x-ray examinations, an image is made by passing x-rays through your body from an outside source. In contrast, nuclear medicine procedures use a radioactive material called a radiotracer, which is injected into your bloodstream, swallowed by mouth or inhaled as a gas. This radioactive material accumulates in the organ or area of your body being examined, where it gives off a small amount of energy in the form of gamma rays. A gamma camera, PET scanner, or probe detect this energy and with the help of a computer create pictures offering details on both the structure and function of organs and other parts of your body.
Unlike other imaging techniques, PET does not produce clear structural images but rather a rendering that reflects the level of chemical activity in your body. Areas of greater intensity, called hot spots, indicate where large amounts of the radiotracer have accumulated and where there is a high level of chemical activity. Less intense areas, or cold spots, indicate a smaller concentration of radiotracer and less chemical activity.
How is it performed?
Nuclear medicine imaging is usually performed on an outpatient basis, but is often performed on hospitalized patients as well.
You will be positioned on an examination table. If necessary, a nurse or technologist will insert an intravenous (IV) line into a vein in your hand or arm.
Depending on the type of nuclear medicine exam you are undergoing, the dose of radiotracer is then injected intravenously, swallowed by mouth or inhaled as a gas.
It will take approximately 30 to 60 minutes for the radiotracer to travel through your body and to be absorbed by the organ or tissue being studied. You will be asked to rest quietly, avoiding movement and talking.
You will then be moved into the PET scanner and the imaging will begin. You will need to remain still during imaging.
Actual scanning time is approximately 45 minutes.
Depending on which organ or tissue is being examined, additional tests involving other tracers or drugs may be used, which could lengthen the procedure time to four hours. For example, if you are being examined for heart disease, you may undergo a PET scan both before and after exercising.
When the examination is completed, you may be asked to wait until the technologist checks the images in case additional images are needed.
If you had an intravenous line inserted for the procedure, it will be removed.
What will I experience during and after the procedure?
Most nuclear medicine procedures are painless.
If the radiotracer is given intravenously, you will feel a slight pin prick when the needle is inserted into your vein for the intravenous line. When the radioactive material is injected into your arm, you may feel a cold sensation moving up your arm, but there are generally no other side effects.
When swallowed, the radiotracer has little or no taste. When inhaled, you should feel no differently than when breathing room air or holding your breath.
With some procedures, a catheter may be placed into your bladder, which may cause temporary discomfort.
It is important that you remain still while the images are being recorded. Though nuclear imaging itself causes no pain, there may be some discomfort from having to remain still or to stay in one particular position during imaging.
If you are claustrophobic, you may feel some anxiety while you are being scanned.
Unless your physician tells you otherwise, you may resume your normal activities after your nuclear medicine scan.
Through the natural process of radioactive decay, the small amount of radiotracer in your body will lose its radioactivity over time. In many cases, the radioactivity will dissipate over the first 24 hours following the test and pass out of your body through your urine or stool. You may be instructed to take special precautions after urinating, to flush the toilet twice and to wash your hands thoroughly. You should also drink plenty of water to help flush the radioactive material out of your body.
Who interprets the results and how do I get them?
A radiologist who has specialized training in nuclear medicine will interpret the images and forward a report to your referring physician.
What are the benefits vs. risks?
Benefits
The information provided by nuclear medicine examinations is unique and often unattainable using other imaging procedures.
For many diseases, nuclear medicine scans yield the most useful information needed to make a diagnosis or to determine appropriate treatment, if any.
Nuclear medicine is much less traumatic than exploratory surgery.
By identifying changes in the body at the cellular level, PET imaging may detect the early onset of disease before it is evident on other imaging tests such as CT or MRI.
Risks
Because the doses of radiotracer administered are small, diagnostic nuclear medicine procedures result in minimal radiation exposure. Thus, the radiation risk is very low compared with the potential benefits.
Nuclear medicine has been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure.
Allergic reactions to radiopharmaceuticals may occur but are extremely rare.
Injection of the radiotracer may cause slight pain and redness which should rapidly resolve.
Women should always inform their physician or radiology technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby. See the Safety page for more information about pregnancy, breastfeeding and nuclear medicine exams.
What are the limitations of Positron Emission Tomography?
Nuclear medicine procedures can be time-consuming. It can take hours to days for the radiotracer to accumulate in the part of the body under study and imaging may take up to several hours to perform, though new equipment is available that can substantially shorten the procedure time.
The resolution of structures of the body with nuclear medicine may not be as clear as with other imaging techniques, such as CT or MRI. However, the information gained from nuclear medicine is unequaled in other imaging techniques.
PET scanning can give false results if chemical balances within the body are not normal. Specifically, test results of diabetic patients or patients who have eaten within a few hours prior to the examination can be adversely affected because of altered blood sugar or blood insulin levels.
Because the radioactive substance decays quickly and is effective for only a short period of time, it is important for the patient to be on time for the appointment and to receive the radioactive substance at the scheduled time. Thus, late arrival for an appointment may require rescheduling the procedure for another day.
http://www.petscaninfo.com/zportal/portals/pat/
What is PET? Positron Emission Tomography (PET) is a powerful imaging technique that holds great promise in the diagnosis and treatment of many diseases, particularly cancer. A non-invasive test, PET scans accurately image the cellular function of the human body. In a single PET scan your physician can examine your entire body. PET scanning provides a more complete picture, making it easier for your doctor to diagnose problems, determine the extent of disease, prescribe treatment, and track progress. What is PET/CT? PET (Positron Emission Tomography) and CT (Computed Tomography) scans are both standard imaging tools that physicians use to pinpoint disease states in the body. A PET scan demonstrates the biological function of the body before anatomical changes take place, while the CT scan provides information about the body's anatomy such as size, shape and location. By combining these two scanning technologies, a PET/CT scan enables physicians to more accurately diagnose and identify cancer, heart disease and brain disorders.
Positron Emission Tomography (PET)
A PET scan allows physicians to measure the body's abnormal molecular cell activity to detect
Cancer (such as breast cancer, lung cancer, colorectal cancer, lymphoma, melanoma and other skin cancers),
Brain Disorders (such as Alzheimer's Disease, Parkinson's Disease, and epilepsy), and
Heart Disease (such as coronary artery disease).
PET scans are simple, painless, and fast, offering patients and their families life-saving information that helps physicians detect and diagnose diseases early and quickly begin treatment.
PET scanning and molecular imaging provide real life answers to better diagnose illness, guide treatment options, and give patients ultimate control over their critical and vital health care decisions.
What is PET?
PET (or positron emission tomography) is a medical imaging tool which assists physicians in detecting disease. Simply stated, PET scans produce digital pictures that can, in many cases, identify many forms of cancer, damaged heart tissue, and brain disorders such as Alzheimer's, Parkinson's, and epilepsy. Technically, PET is a medical imaging technology that images the biology of disorders at the molecular level before anatomical changes are visible.
A PET scan is very different from an ultrasound, X-ray, MRI, or CT, which detect changes in the body structure or anatomy, such as a lesion (for example, a sizeable tumor) or musculoskeletal injury. A PET scan can distinguish between benign and malignant disorders (or between alive and dead tissue), unlike other imaging technologies which merely confirm the presence of a mass.
A PET scan can detect abnormalities in cellular activity generally before there is any anatomical change. A PET scan can, in many cases, identify diseases earlier and more specifically than ultrasound, X-rays, CT, or MRI.
PET can also help physicians monitor the treatment of disease. For example, chemotherapy leads to changes in cellular activity and that is observable by PET long before structural changes can be measured by ultrasound, X-rays, CT, or MRI. A PET scan gives physicians another tool to evaluate treatments, perhaps even leading to a modification in treatment, before an evaluation could be made using other imaging technologies.
How PET Works
When disease strikes, the biochemistry of your tissues and cells change. In cancer, for example, cells begin to grow at a much faster rate. A PET scan takes a digital picture of abnormal cellular structure.
The most common form of a PET scan begins with an injection of a glucose-based radiopharmaceutical (FDG), which travels through the body, eventually collecting in the organs and tissues targeted for examination. The patient lies flat on a bed/table that moves incrementally through the PET scanner. The scanner has cameras that detect the gamma rays emitted from the patient, and turns those into electrical signals, which are processed by a computer to generate the medical images. The bed/table moves a few inches again, and the process is repeated.
This produces the digital images, which are assembled by the computer into a 3-D image of the patient's body. If an area is cancerous, the signals will be stronger there than in surrounding tissue, since more of the radiopharmaceutical (FDG) will be absorbed in those areas.
Why PET Works
PET scans give information about the body's chemistry that is not available with other imaging techniques.
PET scans reveal metabolic information (as opposed to anatomical information), providing your physician with extra insight.
Because PET scanning often reveals disease much earlier than conventional diagnostic procedures (such as CT or MRI), it can help physicians diagnose disease faster.
What PET Sees
PET is a procedure that is able to detect small cancerous tumors, and also subtle changes in the brain and heart. This enables physicians to treat these diseases earlier and more accurately than if they waited for the results from other detection modalities.
A PET scan puts time on your side! The earlier the diagnosis, the better the chance for successful treatment.
PET scans offer patients hope.
PET can detect disease sooner and the earlier the detection, the more likely the cure! Prior to changes in structure that normally would show up on a CT or MRI scan, a PET scan can reveal metabolic changes in the body. Cancer is a metabolic process and PET is a metabolic imaging technique.
PET shows the extent of disease - called staging - of lung cancer, colorectal cancer, melanoma, head and neck cancer, breast cancer, lymphoma and many other cancers. For patients whose cancer is newly diagnosed, it is important to determine if the cancer has spread to other parts of the body so that appropriate treatment can be started. PET can search the entire body for cancer in a single examination with a "whole body scan," revealing the primary site (s) as well as any metastases.
PET shows whether a tumor is benign or malignant. Reports in scientific literature find that, in some tumors, PET correctly identifies detected lesions 95% of the time. Painful, costly and invasive surgery, such as thoracotomy, may no longer be necessary for diagnosis.
PET shows the effectiveness of therapy. It is an excellent way to monitor progress and test recurrence of disease. For example, an ovarian cancer patient with a blood test that indicated a rise in her tumor marker levels had a PET scan after both CT and MRI scans were still registering no cancer. Only the PET scan showed the new cancer. After treatment, a subsequent PET scan revealed that the cancer was gone.
PET Scan For ...
Cancer:
To assess tumor aggressiveness
To monitor success of therapy
To detect early any recurrent tumors
To provide a whole-body survey for cancer that may have spread
To identify benign and malignant growths
Heart Disease:
To determine what heart tissue is still alive following a suspected heart attack
To predict success of angioplasty (balloon) or bypass surgery
To determine if coronary arteries are blocked
Brain Disorders:
To diagnose Alzheimer's and other dementia
To determine the location of epileptic seizures prior to surgery
To diagnose movement disorders like Parkinson's disease
My PET Scan
Difficult questions deserve answers, and taking a "wait and see" approach is sometimes unacceptable. A PET scan helps answer those tough questions.
Your PET scan will produce a "picture" of how your body's cells are functioning, whereas an x-ray, CT scan, or MRI produce a picture of bones, organs and tissues.
Your PET scan will help you and your physician make a more informed decision about your diagnosis and treatment path.
PET Scans and Cancer
PET can help physicians effectively pinpoint the source of cancer. This is possible because many cancer cells are highly metabolic and therefore synthesize the radioactive glucose (sugar) that is injected in the patient prior to the exam. The areas of high glucose uptake are dramatically displayed in the scan imagery, as opposed to the anatomical imagery of CT or MRI, which cannot detect active, viable tumors.
If cancer is found early, it can often be cured. A PET scan can be used in early diagnosis, assisting physicians in determining the best method for treatment. A whole body PET scan may detect whether cancer is isolated to one specific area or has spread to other organs before a treatment path is determined.
Approximately 1,444,920 new cancer cases are expected to be diagnosed in 2007. According to the American Cancer Society, approximately 559,650 Americans are expected to die of cancer this year, more than 1,500 people per day.
What is Cancer?
Cancer comes in a variety of forms. Basically, cancer occurs when cells in the body begin to grow chaotically. Normally, cells grow, divide, and produce more cells to keep the body healthy and functioning properly. Sometimes, however, the process goes astray; cells keep dividing when new cells are not needed. Some types of cells are more prone to abnormal growth than others. The mass of extra cells forms a growth or tumor, which can be benign or malignant.
Benign tumors are not cancer. They often can be removed and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body. More importantly, benign tumors are rarely life threatening.
Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control or order. These cancer cells can invade and destroy the tissue around them. In a process called metastasis, cancerous cells break away from the organs on which they are growing and travel to other parts of the body, where they continue to grow. Cells from cancerous ovaries, for example, commonly spread to the abdomen and nearby internal organs. Eventually, they can invade the bloodstream and lymph system (the two systems of vessels that bathe and feed all of the body's organs) and travel to organs throughout the body. Metastasis is how cancer "colonizes" to produce new tumors within the body.
http://en.wikipedia.org/wiki/PET-CT
Positron emission tomography - computed tomography (better know by its acronym PET-CT) is a medical imaging device which combines in a single gantry system both a Positron Emission Tomography (PET) and an x-ray Computed Tomography, so that images acquired from both devices can be taken sequentially, in the same session from the patient and combined into a single superposed (co-registered) image. Thus, functional imaging obtained by PET, which depicts the spatial distribution of metabolic or biochemical activity in the body can be more precisely aligned or correlated with anatomic imaging obtained by CT scanning. Two- and three-dimensional image reconstruction may be rendered as a function of a common software and control system.
PET-CT has revolutionized many fields of medical diagnosis, by adding precision of anatomic localization to functional imaging, which was previously lacking from pure PET imaging. For example, in oncology, surgical planning, radiation therapy and cancer staging have been changing rapidly under the influence of PET-CT availability, to the extent that many diagnostic imaging procedures and centers have been gradually abandoning conventional PET devices and substituting them by PET-CTs. Although the combined device is considerably more expensive, it has the advantage of providing both functions as stand-alone examinations, being, in fact, two devices in one.
The only other obstacle to a wider dissemination of PET-CT is the difficulty and cost of producing and transporting the radiopharmaceuticals used for PET imaging, which are usually extremely short-lived (for instance, the half life of radioactive fluor18 used to trace glucose metabolism (using fluorodeoxyglucose -- FDG) is two hours only. Its production requires a very expensive synchrotron as well as a production line for the radiopharmaceuticals.
[edit] Procedure for FDG imaging
An example of how PET-CT works in the diagnostic work-up of FDG metabolic mapping follows:
Before the exam, the patient undergoes a minimum of 8-hour fasting and rest;
In the day of the exam, the patient rests lying for a minimum of 15 min, in order to quiet down muscular activity, which might be interpreted as abnormal metabolism;
An intravenous bolus injection of a dose of recently produced 2-FDG or 3-FDG is made, usually by a vein in one of the arms. Dosage ranges from 0,1 to 0,2 mCi per kg of body weight;
After one or two hours, the patient is placed into the PET-CT device, usually lying in a supine position with his/her arms resting at the sides, or brought together above the head, depending on the main region of interest (ROI)
An automatic bed moves head first into the gantry, first obtaining a topogram, also called a scout view, which is a kind of whole body flat sagital section, obtained with the X-ray tube fixed into the upper position.
The operator uses the PET-CT computer console to identify the patient and examination, delimit the caudal and rostral limits of the body scan onto the scout view, selects the scanning parameters and starts the image acquisition period, which follows without human intervention;
The patient is automatically moved head first into the CT gantry, and the x-ray tomogram is acquired;
Now the patient is automatically moved through the PET gantry, which is mounted in parallel with the CT gantry, and the PET slices are acquired;
The patient may now leave the device, and the PET-CT software starts reconstructing and aligning the PET and CT images.
A whole body scan, which usually is made from mid-thights to the top of the head, takes about 40 min. FDG imaging protocols acquires slices with a thickness of 2 to 3 mm. Hypermetabolic lesions are shown as false color-coded pixels or voxels onto the gray-value coded CT images. Standard Uptake Values are calculated by the software for each hypermetabolic region detected in the image. It provides a quantification of size of the lesion, since functional imaging does not provide a precise anatomical estimate of its extent. The CT can be used for that, when the lesion is also visualized in its images (this is not always the case when hypermetabolic lesions are not accompanied by anatomical changes).
FDG doses in quantities sufficient to carry out 4-5 examinations are delivered daily, twice or more times per day, by the provider to the diagnostic imaging center.
For uses in stereotactic radiation therapy of cancer, special fiducial marks are placed in the patient's body before acquiring the PET-CT images. The slices thus acquired may be transferred digitally to a linear accelerator which is used to perform precise bombardment of the target areas using high energy photons (radiosurgery).
PET
http://en.wikipedia.org/wiki/Positron_emission_tomography
Positron emission tomography
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Image of a typical positron emission tomography (PET) facility
Positron emission tomography (PET) is a nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule. Images of tracer concentration in 3-dimensional space within the body are then reconstructed by computer analysis. In modern scanners, this reconstruction is often accomplished with the aid of a CT X-ray scan performed on the patient during the same session, in the same machine.
If the biologically active molecule chosen for PET is FDG, an analogue of glucose, the concentrations of tracer imaged then give tissue metabolic activity, in terms of regional glucose uptake. Although use of this tracer results in the most common type of PET scan, other tracer molecules are used in PET to image the tissue concentration of many other types of molecules of interest.
Contents
[hide]
1 Description
1.1 Operation
1.2 Localization of the positron annihilation event
1.3 Image reconstruction using coincidence statistics
1.4 Combination of PET with CT and MRI
1.5 Radioisotopes
1.6 Limitations
1.7 Image reconstruction
2 History
3 Applications
4 Safety
5 See also
6 References
7 Further reading
8 External links
//
[edit] Description
Schematic view of a detector block and ring of a PET scanner
[edit] Operation
To conduct the scan, a short-lived radioactive tracer isotope, which decays by emitting a positron, which also has been chemically incorporated into a biologically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the active molecule becomes concentrated in tissues of interest; then the research subject or patient is placed in the imaging scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour.
Schema of a PET acquisition process
As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, the antimatter counterpart of an electron. After travelling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of annihilation (gamma) photons moving in opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes or silicon avalanche photodiodes (Si APD). The technique depends on simultaneous or coincident detection of the pair of photons; photons which do not arrive in pairs (i.e. within a few nanoseconds) are ignored.
[edit] Localization of the positron annihilation event
The most significant fraction of electron-positron decays result in two 511 keV gamma photons being emitted at almost 180 degrees to each other; hence it is possible to localize their source along a straight line of coincidence (also called formally the line of response or LOR). In practice the LOR has a finite width as the emitted photons are not exactly 180 degrees apart. If the recovery time of detectors is in the picosecond range rather than the 10's of nanosecond range, it is possible to calculate the single point on the LOR at which an annihilation event originated, by measuring the "time of flight" of the two photons. This technology is not yet common, but it is available on some new systems [1].
[edit] Image reconstruction using coincidence statistics
More commonly, a technique much like the reconstruction of computed tomography (CT) and single photon emission computed tomography (SPECT) data is used, although the data set collected in PET is much poorer than CT, so reconstruction techniques are more difficult (see Image reconstruction of PET).
Using statistics collected from tens-of-thousands of coincidence events, a set of simultaneous equations for the total activity of each parcel of tissue along many LORs can be solved by a number of techniques, and thus a map of radioactivities as a function of location for parcels or bits of tissue (also called voxels), may be constructed and plotted. The resulting map shows the tissues in which the molecular probe has become concentrated, and can be interpreted by a nuclear medicine physician or radiologist in the context of the patient's diagnosis and treatment plan.
A complete body PET / CT Fusion image
A Brain PET / MRI Fusion image
[edit] Combination of PET with CT and MRI
PET scans are increasingly read alongside CT or magnetic resonance imaging (MRI) scans, the combination ("co-registration") giving both anatomic and metabolic information (i.e., what the structure is, and what it is doing biochemically). Because PET imaging is most useful in combination with anatomical imaging, such as CT, modern PET scanners are now available with integrated high-end multi-detector-row CT scanners. Because the two scans can be performed in immediate sequence during the same session, with the patient not changing position between the two types of scans, the two sets of images are more-precisely registered, so that areas of abnormality on the PET imaging can be more perfectly correlated with anatomy on the CT images. This is very useful in showing detailed views of moving organs or structures with higher amounts of anatomical variation, such as are more likely to occur outside the brain.
[edit] Radioisotopes
Radionuclides used in PET scanning are typically isotopes with short half lives such as carbon-11 (~20 min), nitrogen-13 (~10 min), oxygen-15 (~2 min), and fluorine-18 (~110 min). These radionuclides are incorporated either into compounds normally used by the body such as glucose (or glucose analogues), water or ammonia, or into molecules that bind to receptors or other sites of drug action. Such labelled compounds are known as radiotracers. It is important to recognize that PET technology can be used to trace the biologic pathway of any compound in living humans (and many other species as well), provided it can be radiolabeled with a PET isotope. Thus the specific processes that can be probed with PET are virtually limitless, and radiotracers for new target molecules and processes are being synthesized all the time; as of this writing there are already dozens in clinical use and hundreds applied in research. Due to the short half lives of most radioisotopes, the radiotracers must be produced using a cyclotron and radiochemistry laboratory that are in close proximity to the PET imaging facility. The half life of fluorine-18 is long enough such that fluorine-18 labeled radiotracers can be manufactured commercially at an offsite location.
[edit] Limitations
The minimization of radiation dose to the subject is an attractive feature of the use of short-lived radionuclides. Besides its established role as a diagnostic technique, PET has an expanding role as a method to assess the response to therapy, in particular, cancer therapy,[1] where the risk to the patient from lack of knowledge about disease progress is much greater than the risk from the test radiation.
Limitations to the widespread use of PET arise from the high costs of cyclotrons needed to produce the short-lived radionuclides for PET scanning and the need for specially adapted on-site chemical synthesis apparatus to produce the radiopharmaceuticals. Few hospitals and universities are capable of maintaining such systems, and most clinical PET is supported by third-party suppliers of radiotracers which can supply many sites simultaneously. This limitation restricts clinical PET primarily to the use of tracers labelled with F-18, which has a half life of 110 minutes and can be transported a reasonable distance before use, or to rubidium-82, which can be created in a portable generator and is used for myocardial perfusion studies. Nevertheless, in recent years a few on-site cyclotrons with integrated shielding and hot labs have begun to accompany PET units to remote hospitals. The presence of the small on-site cyclotron promises to expand in the future as the cyclotrons shrink in response to the high cost of isotope transportation to remote PET machines [2]
Because the half-life of F-18 is about two hours, the prepared dose of a radiopharmaceutical bearing this radionuclide will undergo multiple half-lives of decay during the working day. This necessitates frequent recalibration of the remaining dose (determination of activity per unit volume) and careful planning with respect to patient scheduling.
[edit] Image reconstruction
The raw data collected by a PET scanner are a list of 'coincidence events' representing near-simultaneous detection of annihilation photons by a pair of detectors. Each coincidence event represents a line in space connecting the two detectors along which the positron emission occurred.
Coincidence events can be grouped into projections images, called sinograms. The sinograms are sorted by the angle of each view and tilt, the latter in 3D case images. The sinogram images are analogous to the projections captured by computed tomography (CT) scanners, and can be reconstructed in a similar way. However, the statistics of the data is much worse than those obtained through transmission tomography. A normal PET data set has millions of counts for the whole acquisition, while the CT can reach a few billion counts. As such, PET data suffer from scatter and random events much more dramatically than CT data does.
In practice, considerable pre-processing of the data is required - correction for random coincidences, estimation and subtraction of scattered photons, detector dead-time correction (after the detection of a photon, the detector must "cool down" again) and detector-sensitivity correction (for both inherent detector sensitivity and changes in sensitivity due to angle of incidence).
Filtered back projection (FBP) has been frequently used to reconstruct images from the projections. This algorithm has the advantage of being simple while having a low requirement for computing resources. However, shot noise in the raw data is prominent in the reconstructed images and areas of high tracer uptake tend to form streaks across the image.
Iterative expectation-maximization algorithms are now the preferred method of reconstruction. The advantage is a better noise profile and resistance to the streak artifacts common with FBP, but the disadvantage is higher computer resource requirements.
Attenuation correction: As different LORs must traverse different thicknesses of tissue, the photons are attenuated differentially. The result is that structures deep in the body are reconstructed as having falsely low tracer uptake. Contemporary scanners can estimate attenuation using integrated x-ray CT equipment, however earlier equipment offered a crude form of CT using a gamma ray (positron emitting) source and the PET detectors.
While attenuation corrected images are generally more faithful representations, the correction process is itself susceptible to significant artifacts. As a result, both corrected and uncorrected images are always reconstructed and read together.
2D/3D reconstruction: Early PET scanners had only a single ring of detectors, hence the acquisition of data and subsequent reconstruction was restricted to a single transverse plane. More modern scanners now include multiple rings, essentially forming a cylinder of detectors.
There are two approaches to reconstructing data from such a scanner: 1) treat each ring as a separate entity, so that only coincidences within a ring are detected, the image from each ring can then be reconstructed individually (2D reconstruction), or 2) allow coincidences to be detected between rings as well as within rings, then reconstruct the entire volume together (3D).
3D techniques have better sensitivity (because more coincidences are detected and used) and therefore less noise, but are more sensitive to the effects of scatter and random coincidences, as well as requiring correspondingly greater computer resources.
[edit] History
The concept of emission and transmission tomography was introduced by David Kuhl and Roy Edwards in the late 1950s. Their work later led to the design and construction of several tomographic instruments at the University of Pennsylvania. Tomographic imaging techniques were further developed by Michel Ter-Pogossian, Michael E. Phelps and others at the Washington University School of Medicine.[3][4]
In the 1970s, Tatsuo Ido at the Brookhaven National Laboratory was the first to describe the synthesis of 18F-FDG, the most commonly used PET scanning isotope carrier. The compound was first administered to two normal human volunteers by Abass Alavi in August 1976 at the University of Pennsylvania. Brain images obtained with an ordinary (non-PET) nuclear scanner demonstrated the concentration of FDG in that organ. Later, the substance was used in dedicated positron tomographic scanners, to yield the modern procedure.
[edit] Applications
Maximum intensity projection (MIP) of a typical F-18 FDG wholebody PET acquisition
PET is both a medical and research tool. It is used heavily in clinical oncology (medical imaging of tumors and the search for metastases), and for clinical diagnosis of certain diffuse brain diseases such as those causing various types of dementias. PET is also an important research tool to map normal human brain and heart function.
PET is also used in pre-clinical studies using animals, where it allows repeated investigations into the same subjects. This is particularly valuable in cancer research, as it results in an increase in the statistical quality of the data (subjects can act as their own control) and substantially reduces the numbers of animals required for a given study.
Alternative methods of scanning include x-ray computed tomography (CT), magnetic resonance imaging (MRI) and functional magnetic resonance imaging (fMRI), ultrasound and single photon emission computed tomography (SPECT).
While some imaging scans such as CT and MRI isolate organic anatomic changes in the body, PET and SPECT are capable of detecting areas of molecular biology detail (even prior to anatomic change). PET scanning does this using radiolabelled molecular probes that have different rates of uptake depending on the type and function of tissue involved. Changing of regional blood flow in various anatomic structures (as a measure of the injected positron emitter) can be visualized and relatively quantified with a PET scan.
PET imaging is best performed using a dedicated PET scanner. However, it is possible to acquire PET images using a conventional dual-head gamma camera fitted with a coincidence detector. The quality of gamma-camera PET is considerably lower, and acquisition is slower. However, for institutions with low demand for PET, this may allow on-site imaging, instead of referring patients to another center, or relying on a visit by a mobile scanner.
PET is a valuable technique for some diseases and disorders, because it is possible to target the radio-chemicals used for particular bodily functions.
Oncology: PET scanning with the tracer fluorine-18 (F-18) fluorodeoxyglucose (FDG), called FDG-PET, is widely used in clinical oncology. This tracer is a glucose analog that is taken up by glucose-using cells and phosphorylated by hexokinase (whose mitochondrial form is greatly elevated in rapidly-growing malignant tumours). A typical dose of FDG used in an oncological scan is 200-400 MBq for an adult human. Because the oxygen atom which is replaced by F-18 to generate FDG is required for the next step in glucose metabolism in all cells, no further reactions occur in FDG. Furthermore, most tissues (with the notable exception of liver and kidneys) cannot remove the phosphate added by hexokinase. This means that FDG is trapped in any cell which takes it up, until it decays, since phosphorylated sugars, due to their ionic charge, cannot exit from the cell. This results in intense radiolabeling of tissues with high glucose uptake, such as the brain, the liver, and most cancers. As a result, FDG-PET can be used for diagnosis, staging, and monitoring treatment of cancers, particularly in Hodgkin's disease, non Hodgkin's lymphoma, and lung cancer. Many other types of solid tumors will be found to be very highly labeled on a case-by-case basis-- a fact which becomes especially useful in searching for tumor metastasis, or for recurrence after a known highly-active primary tumor is removed. Because individual PET scans are more expensive than "conventional" imaging with computed tomography (CT) and magnetic resonance imaging (MRI), expansion of FDG-PET in cost-constrained health services will depend on proper health technology assessment; this problem is a difficult one because structural and functional imaging often cannot be directly compared, as they provide different information. Oncology scans using FDG make up over 90% of all PET scans in current practice.
PET scan of the human brain.
Neurology: PET neuroimaging is based on an assumption that areas of high radioactivity are associated with brain activity. What is actually measured indirectly is the flow of blood to different parts of the brain, which is generally believed to be correlated, and has been measured using the tracer oxygen-15. However, because of its 2-minute half-life O-15 must be piped directly from a medical cyclotron for such uses, and this is difficult. In practice, since the brain is normally a rapid user of glucose, and since brain pathologies such as Alzheimer's disease greatly decrease brain metabolism of both glucose and oxygen in tandem, standard FDG-PET of the brain, which measures regional glucose use, may also be successfully used to differentiate Alzheimer's disease from other dementing processes, and also to make early diagnosis of Alzheimer's disease. The advantage of FDG-PET for these uses is its much wider availability. PET imaging with FDG can also be used for localization of seizure focus: A seizure focus will appear as hypometabolic during an interictal scan. Several radiotracers (i.e. radioligands) have been developed for PET that are ligands for specific neuroreceptor subtypes such as [11C] raclopride and [18F] fallypride for dopamine D2/D3 receptors, [11C]McN 5652 and [11C]DASB for serotonin transporters, or enzyme substrates (e.g. 6-FDOPA for the AADC enzyme). These agents permit the visualization of neuroreceptor pools in the context of a plurality of neuropsychiatric and neurologic illnesses. A novel probe developed at the University of Pittsburgh termed PIB (Pittsburgh Compound-B) permits the visualization of amyloid plaques in the brains of Alzheimer's patients. This technology could assist clinicians in making a positive clinical diagnosis of AD pre-mortem and aid in the development of novel anti-amyloid therapies.
Cardiology, atherosclerosis and vascular disease study: In clinical cardiology, FDG-PET can identify so-called "hibernating myocardium", but its cost-effectiveness in this role versus SPECT is unclear. Recently, a role has been suggested for FDG-PET imaging of atherosclerosis to detect patients at risk of stroke [2].
Neuropsychology / Cognitive neuroscience: To examine links between specific psychological processes or disorders and brain activity.
Psychiatry: Numerous compounds that bind selectively to neuroreceptors of interest in biological psychiatry have been radiolabeled with C-11 or F-18. Radioligands that bind to dopamine receptors (D1,D2, reuptake transporter), serotonin receptors (5HT1A, 5HT2A, reuptake transporter) opioid receptors (mu) and other sites have been used successfully in studies with human subjects. Studies have been performed examining the state of these receptors in patients compared to healthy controls in schizophrenia, substance abuse, mood disorders and other psychiatric conditions.
Pharmacology: In pre-clinical trials, it is possible to radiolabel a new drug and inject it into animals. The uptake of the drug, the tissues in which it concentrates, and its eventual elimination, can be monitored far more quickly and cost effectively than the older technique of killing and dissecting the animals to discover the same information. PET scanners for rats and non-human primates are marketed for this purpose. The technique is still generally too expensive for the veterinary medicine market, however, so very few pet PET scans are done. Drug occupancy at the purported site of action can also be inferred indirectly by competition studies between unlabeled drug and radiolabeled compounds known apriori to bind with specificity to the site.
[edit] Safety
PET scanning is non-invasive, but it does involve exposure to ionizing radiation. The total dose of radiation is small, however, usually around 7 mSv. This can be compared to 2.2 mSv average annual background radiation in the UK, 0.02 mSv for a chest x-ray, up to 8 mSv for a CT scan of the chest, 2-6 mSv per annum for aircrew (data from UK National Radiological Protection Board). Patients with small children may be advised to limit proximity to them for several hours following the completion of the test.
http://www.radiologyinfo.org/en/info.cfm?pg=pet&bhcp=1
Positron Emission Tomography (PET) Scanning
What is Positron Emission Tomography (PET) Scanning?
What are some common uses of the procedure?
How should I prepare for the procedure?
What does the equipment look like?
How does the procedure work?
How is the procedure performed?
What will I experience during and after procedure?
Who interprets the results and how do I get them?
What are the benefits vs. risks?
What are the limitations of PET?
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What is Positron Emission Tomography (PET) Scanning
Sample image obtained using a combination of PET and CT imaging technology.
Positron emission tomography, also called PET imaging or a PET scan, is a type of nuclear medicine imaging.
Nuclear medicine is a subspecialty within the field of radiology that uses very small amounts of radioactive material to diagnose or treat disease and other abnormalities within the body.
Nuclear medicine imaging procedures are noninvasive and usually painless medical tests that help physicians diagnose medical conditions. These imaging scans use radioactive materials called a radiopharmaceutical or radiotracer.
Depending on the type of nuclear medicine exam you are undergoing, the radiotracer is injected into a vein, swallowed by mouth or inhaled as a gas and eventually collects in the area of your body being examined, where it gives off energy in the form of gamma rays. This energy is detected by a device called a gamma camera, a (positron emission tomography) PET scanner and/or probe. These devices work together with a computer to measure the amount of radiotracer absorbed by your body and to produce special pictures offering details on both the structure and function of organs and other internal body parts.
In some centers, nuclear medicine images can be superimposed with computed tomography (CT) or magnetic resonance imaging (MRI) to produce special views, a practice known as image fusion or co-registration. These views allow the information from two different studies to be correlated and interpreted on one image, leading to more precise information and accurate diagnoses.
A PET scan measures important body functions, such as blood flow, oxygen use, and sugar (glucose) metabolism, to help doctors evaluate how well organs and tissues are functioning.
What are some common uses of the procedure?
PET scans are performed to:
detect cancer
determine how much a cancer has spread in the body
assess the effectiveness of a treatment plan, such as cancer therapy
determine if a cancer has returned after treatment
determine blood flow to the heart muscle
determine the effects of a heart attack, or myocardial infarction, on areas of the heart
identify areas of the heart muscle that would benefit from a procedure such as angioplasty or coronary artery bypass surgery (in combination with a myocardial perfusion scan).
evaluate brain abnormalities, such as tumors, memory disorders and seizures and other central nervous system disorders
to map normal human brain and heart function
How should I prepare?
You may be asked to wear a gown during the exam or you may be allowed to wear your own clothing.
Women should always inform their physician or technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby. See the Safety page for more information about pregnancy and breastfeeding related to nuclear medicine imaging.
You should inform your physician of any medications you are taking as well as vitamins and herbal supplements and if you have any allergies. Also inform your doctor about recent illnesses or other medical conditions.
Jewelry and other accessories should be left at home if possible, or removed prior to the exam because they may interfere with the procedure.
You will receive specific instructions based on the type of PET scan you are undergoing. Diabetic patients will receive special instructions to prepare for this exam.
What does the equipment look like?
A positron emission tomography (PET) scanner is a large machine with a round, doughnut shaped hole in the middle, similar to a CT unit. Within this machine are multiple rings of detectors that record the emission of energy from the radiotracer in your body.
A nearby computer aids in creating the images from the data obtained by the camera or scanner.
How does the procedure work?
With ordinary x-ray examinations, an image is made by passing x-rays through your body from an outside source. In contrast, nuclear medicine procedures use a radioactive material called a radiotracer, which is injected into your bloodstream, swallowed by mouth or inhaled as a gas. This radioactive material accumulates in the organ or area of your body being examined, where it gives off a small amount of energy in the form of gamma rays. A gamma camera, PET scanner, or probe detect this energy and with the help of a computer create pictures offering details on both the structure and function of organs and other parts of your body.
Unlike other imaging techniques, PET does not produce clear structural images but rather a rendering that reflects the level of chemical activity in your body. Areas of greater intensity, called hot spots, indicate where large amounts of the radiotracer have accumulated and where there is a high level of chemical activity. Less intense areas, or cold spots, indicate a smaller concentration of radiotracer and less chemical activity.
How is it performed?
Nuclear medicine imaging is usually performed on an outpatient basis, but is often performed on hospitalized patients as well.
You will be positioned on an examination table. If necessary, a nurse or technologist will insert an intravenous (IV) line into a vein in your hand or arm.
Depending on the type of nuclear medicine exam you are undergoing, the dose of radiotracer is then injected intravenously, swallowed by mouth or inhaled as a gas.
It will take approximately 30 to 60 minutes for the radiotracer to travel through your body and to be absorbed by the organ or tissue being studied. You will be asked to rest quietly, avoiding movement and talking.
You will then be moved into the PET scanner and the imaging will begin. You will need to remain still during imaging.
Actual scanning time is approximately 45 minutes.
Depending on which organ or tissue is being examined, additional tests involving other tracers or drugs may be used, which could lengthen the procedure time to four hours. For example, if you are being examined for heart disease, you may undergo a PET scan both before and after exercising.
When the examination is completed, you may be asked to wait until the technologist checks the images in case additional images are needed.
If you had an intravenous line inserted for the procedure, it will be removed.
What will I experience during and after the procedure?
Most nuclear medicine procedures are painless.
If the radiotracer is given intravenously, you will feel a slight pin prick when the needle is inserted into your vein for the intravenous line. When the radioactive material is injected into your arm, you may feel a cold sensation moving up your arm, but there are generally no other side effects.
When swallowed, the radiotracer has little or no taste. When inhaled, you should feel no differently than when breathing room air or holding your breath.
With some procedures, a catheter may be placed into your bladder, which may cause temporary discomfort.
It is important that you remain still while the images are being recorded. Though nuclear imaging itself causes no pain, there may be some discomfort from having to remain still or to stay in one particular position during imaging.
If you are claustrophobic, you may feel some anxiety while you are being scanned.
Unless your physician tells you otherwise, you may resume your normal activities after your nuclear medicine scan.
Through the natural process of radioactive decay, the small amount of radiotracer in your body will lose its radioactivity over time. In many cases, the radioactivity will dissipate over the first 24 hours following the test and pass out of your body through your urine or stool. You may be instructed to take special precautions after urinating, to flush the toilet twice and to wash your hands thoroughly. You should also drink plenty of water to help flush the radioactive material out of your body.
Who interprets the results and how do I get them?
A radiologist who has specialized training in nuclear medicine will interpret the images and forward a report to your referring physician.
What are the benefits vs. risks?
Benefits
The information provided by nuclear medicine examinations is unique and often unattainable using other imaging procedures.
For many diseases, nuclear medicine scans yield the most useful information needed to make a diagnosis or to determine appropriate treatment, if any.
Nuclear medicine is much less traumatic than exploratory surgery.
By identifying changes in the body at the cellular level, PET imaging may detect the early onset of disease before it is evident on other imaging tests such as CT or MRI.
Risks
Because the doses of radiotracer administered are small, diagnostic nuclear medicine procedures result in minimal radiation exposure. Thus, the radiation risk is very low compared with the potential benefits.
Nuclear medicine has been used for more than five decades, and there are no known long-term adverse effects from such low-dose exposure.
Allergic reactions to radiopharmaceuticals may occur but are extremely rare.
Injection of the radiotracer may cause slight pain and redness which should rapidly resolve.
Women should always inform their physician or radiology technologist if there is any possibility that they are pregnant or if they are breastfeeding their baby. See the Safety page for more information about pregnancy, breastfeeding and nuclear medicine exams.
What are the limitations of Positron Emission Tomography?
Nuclear medicine procedures can be time-consuming. It can take hours to days for the radiotracer to accumulate in the part of the body under study and imaging may take up to several hours to perform, though new equipment is available that can substantially shorten the procedure time.
The resolution of structures of the body with nuclear medicine may not be as clear as with other imaging techniques, such as CT or MRI. However, the information gained from nuclear medicine is unequaled in other imaging techniques.
PET scanning can give false results if chemical balances within the body are not normal. Specifically, test results of diabetic patients or patients who have eaten within a few hours prior to the examination can be adversely affected because of altered blood sugar or blood insulin levels.
Because the radioactive substance decays quickly and is effective for only a short period of time, it is important for the patient to be on time for the appointment and to receive the radioactive substance at the scheduled time. Thus, late arrival for an appointment may require rescheduling the procedure for another day.
http://www.petscaninfo.com/zportal/portals/pat/
What is PET? Positron Emission Tomography (PET) is a powerful imaging technique that holds great promise in the diagnosis and treatment of many diseases, particularly cancer. A non-invasive test, PET scans accurately image the cellular function of the human body. In a single PET scan your physician can examine your entire body. PET scanning provides a more complete picture, making it easier for your doctor to diagnose problems, determine the extent of disease, prescribe treatment, and track progress. What is PET/CT? PET (Positron Emission Tomography) and CT (Computed Tomography) scans are both standard imaging tools that physicians use to pinpoint disease states in the body. A PET scan demonstrates the biological function of the body before anatomical changes take place, while the CT scan provides information about the body's anatomy such as size, shape and location. By combining these two scanning technologies, a PET/CT scan enables physicians to more accurately diagnose and identify cancer, heart disease and brain disorders.
Positron Emission Tomography (PET)
A PET scan allows physicians to measure the body's abnormal molecular cell activity to detect
Cancer (such as breast cancer, lung cancer, colorectal cancer, lymphoma, melanoma and other skin cancers),
Brain Disorders (such as Alzheimer's Disease, Parkinson's Disease, and epilepsy), and
Heart Disease (such as coronary artery disease).
PET scans are simple, painless, and fast, offering patients and their families life-saving information that helps physicians detect and diagnose diseases early and quickly begin treatment.
PET scanning and molecular imaging provide real life answers to better diagnose illness, guide treatment options, and give patients ultimate control over their critical and vital health care decisions.
What is PET?
PET (or positron emission tomography) is a medical imaging tool which assists physicians in detecting disease. Simply stated, PET scans produce digital pictures that can, in many cases, identify many forms of cancer, damaged heart tissue, and brain disorders such as Alzheimer's, Parkinson's, and epilepsy. Technically, PET is a medical imaging technology that images the biology of disorders at the molecular level before anatomical changes are visible.
A PET scan is very different from an ultrasound, X-ray, MRI, or CT, which detect changes in the body structure or anatomy, such as a lesion (for example, a sizeable tumor) or musculoskeletal injury. A PET scan can distinguish between benign and malignant disorders (or between alive and dead tissue), unlike other imaging technologies which merely confirm the presence of a mass.
A PET scan can detect abnormalities in cellular activity generally before there is any anatomical change. A PET scan can, in many cases, identify diseases earlier and more specifically than ultrasound, X-rays, CT, or MRI.
PET can also help physicians monitor the treatment of disease. For example, chemotherapy leads to changes in cellular activity and that is observable by PET long before structural changes can be measured by ultrasound, X-rays, CT, or MRI. A PET scan gives physicians another tool to evaluate treatments, perhaps even leading to a modification in treatment, before an evaluation could be made using other imaging technologies.
How PET Works
When disease strikes, the biochemistry of your tissues and cells change. In cancer, for example, cells begin to grow at a much faster rate. A PET scan takes a digital picture of abnormal cellular structure.
The most common form of a PET scan begins with an injection of a glucose-based radiopharmaceutical (FDG), which travels through the body, eventually collecting in the organs and tissues targeted for examination. The patient lies flat on a bed/table that moves incrementally through the PET scanner. The scanner has cameras that detect the gamma rays emitted from the patient, and turns those into electrical signals, which are processed by a computer to generate the medical images. The bed/table moves a few inches again, and the process is repeated.
This produces the digital images, which are assembled by the computer into a 3-D image of the patient's body. If an area is cancerous, the signals will be stronger there than in surrounding tissue, since more of the radiopharmaceutical (FDG) will be absorbed in those areas.
Why PET Works
PET scans give information about the body's chemistry that is not available with other imaging techniques.
PET scans reveal metabolic information (as opposed to anatomical information), providing your physician with extra insight.
Because PET scanning often reveals disease much earlier than conventional diagnostic procedures (such as CT or MRI), it can help physicians diagnose disease faster.
What PET Sees
PET is a procedure that is able to detect small cancerous tumors, and also subtle changes in the brain and heart. This enables physicians to treat these diseases earlier and more accurately than if they waited for the results from other detection modalities.
A PET scan puts time on your side! The earlier the diagnosis, the better the chance for successful treatment.
PET scans offer patients hope.
PET can detect disease sooner and the earlier the detection, the more likely the cure! Prior to changes in structure that normally would show up on a CT or MRI scan, a PET scan can reveal metabolic changes in the body. Cancer is a metabolic process and PET is a metabolic imaging technique.
PET shows the extent of disease - called staging - of lung cancer, colorectal cancer, melanoma, head and neck cancer, breast cancer, lymphoma and many other cancers. For patients whose cancer is newly diagnosed, it is important to determine if the cancer has spread to other parts of the body so that appropriate treatment can be started. PET can search the entire body for cancer in a single examination with a "whole body scan," revealing the primary site (s) as well as any metastases.
PET shows whether a tumor is benign or malignant. Reports in scientific literature find that, in some tumors, PET correctly identifies detected lesions 95% of the time. Painful, costly and invasive surgery, such as thoracotomy, may no longer be necessary for diagnosis.
PET shows the effectiveness of therapy. It is an excellent way to monitor progress and test recurrence of disease. For example, an ovarian cancer patient with a blood test that indicated a rise in her tumor marker levels had a PET scan after both CT and MRI scans were still registering no cancer. Only the PET scan showed the new cancer. After treatment, a subsequent PET scan revealed that the cancer was gone.
PET Scan For ...
Cancer:
To assess tumor aggressiveness
To monitor success of therapy
To detect early any recurrent tumors
To provide a whole-body survey for cancer that may have spread
To identify benign and malignant growths
Heart Disease:
To determine what heart tissue is still alive following a suspected heart attack
To predict success of angioplasty (balloon) or bypass surgery
To determine if coronary arteries are blocked
Brain Disorders:
To diagnose Alzheimer's and other dementia
To determine the location of epileptic seizures prior to surgery
To diagnose movement disorders like Parkinson's disease
My PET Scan
Difficult questions deserve answers, and taking a "wait and see" approach is sometimes unacceptable. A PET scan helps answer those tough questions.
Your PET scan will produce a "picture" of how your body's cells are functioning, whereas an x-ray, CT scan, or MRI produce a picture of bones, organs and tissues.
Your PET scan will help you and your physician make a more informed decision about your diagnosis and treatment path.
PET Scans and Cancer
PET can help physicians effectively pinpoint the source of cancer. This is possible because many cancer cells are highly metabolic and therefore synthesize the radioactive glucose (sugar) that is injected in the patient prior to the exam. The areas of high glucose uptake are dramatically displayed in the scan imagery, as opposed to the anatomical imagery of CT or MRI, which cannot detect active, viable tumors.
If cancer is found early, it can often be cured. A PET scan can be used in early diagnosis, assisting physicians in determining the best method for treatment. A whole body PET scan may detect whether cancer is isolated to one specific area or has spread to other organs before a treatment path is determined.
Approximately 1,444,920 new cancer cases are expected to be diagnosed in 2007. According to the American Cancer Society, approximately 559,650 Americans are expected to die of cancer this year, more than 1,500 people per day.
What is Cancer?
Cancer comes in a variety of forms. Basically, cancer occurs when cells in the body begin to grow chaotically. Normally, cells grow, divide, and produce more cells to keep the body healthy and functioning properly. Sometimes, however, the process goes astray; cells keep dividing when new cells are not needed. Some types of cells are more prone to abnormal growth than others. The mass of extra cells forms a growth or tumor, which can be benign or malignant.
Benign tumors are not cancer. They often can be removed and, in most cases, they do not come back. Cells in benign tumors do not spread to other parts of the body. More importantly, benign tumors are rarely life threatening.
Malignant tumors are cancer. Cells in malignant tumors are abnormal and divide without control or order. These cancer cells can invade and destroy the tissue around them. In a process called metastasis, cancerous cells break away from the organs on which they are growing and travel to other parts of the body, where they continue to grow. Cells from cancerous ovaries, for example, commonly spread to the abdomen and nearby internal organs. Eventually, they can invade the bloodstream and lymph system (the two systems of vessels that bathe and feed all of the body's organs) and travel to organs throughout the body. Metastasis is how cancer "colonizes" to produce new tumors within the body.
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